Как передать переменную из одного класса в другой?

Поменяйте импорты на свои и попробуйте.

Как вариант:

main.py

'''
from PyQt6.QtWidgets import *
from PyQt6 import QtCore, QtGui, QtWidgets
'''
import sys
from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.Qt import *

#from стол import Ui_Form as Ui_Form2
class Ui_Form2(object):
    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(685, 465)
        Form.setStyleSheet("background-color: rgb(213, 213, 213);")
        self.label = QtWidgets.QLabel(parent=Form)
        self.label.setGeometry(QtCore.QRect(130, 80, 501, 71))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(28)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.label.setFont(font)
        self.label.setObjectName("label")
        self.pushButton = QtWidgets.QPushButton(parent=Form)
        self.pushButton.setGeometry(QtCore.QRect(220, 340, 261, 51))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(12)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.pushButton.setFont(font)
        self.pushButton.setStyleSheet("background-color: rgb(189, 189, 189);")
        self.pushButton.setObjectName("pushButton")
        self.lineEdit = QtWidgets.QLineEdit(parent=Form)
        self.lineEdit.setGeometry(QtCore.QRect(90, 200, 501, 71))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(16)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.lineEdit.setFont(font)
        self.lineEdit.setObjectName("lineEdit")

        self.retranslateUi(Form)
        QtCore.QMetaObject.connectSlotsByName(Form)

    def retranslateUi(self, Form):
        _translate = QtCore.QCoreApplication.translate
        Form.setWindowTitle(_translate("Form", "Form"))
        self.label.setText(_translate("Form", "Введите номер стола"))
        self.pushButton.setText(_translate("Form", "Перейти к заказу"))
        

#from заказ import Ui_Form as Ui_Form3
class Ui_Form3(object):
    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(1106, 884)
        Form.setStyleSheet("\n"
"background-color: rgb(213, 213, 213);\n"
"")
        self.label = QtWidgets.QLabel(parent=Form)
        self.label.setGeometry(QtCore.QRect(0, -10, 1111, 31))
        self.label.setObjectName("label")
        self.label_2 = QtWidgets.QLabel(parent=Form)
        self.label_2.setGeometry(QtCore.QRect(-20, 30, 1111, 31))
        self.label_2.setObjectName("label_2")
        self.label_3 = QtWidgets.QLabel(parent=Form)
        self.label_3.setGeometry(QtCore.QRect(10, 10, 131, 31))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(18)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.label_3.setFont(font)
        self.label_3.setObjectName("label_3")
        self.pushButton = QtWidgets.QPushButton(parent=Form)
        self.pushButton.setGeometry(QtCore.QRect(500, 110, 171, 101))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(18)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.pushButton.setFont(font)
        self.pushButton.setStyleSheet("background-color: rgb(193, 193, 193);")
        self.pushButton.setObjectName("pushButton")
        self.pushButton_2 = QtWidgets.QPushButton(parent=Form)
        self.pushButton_2.setGeometry(QtCore.QRect(690, 110, 171, 101))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(18)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.pushButton_2.setFont(font)
        self.pushButton_2.setStyleSheet("background-color: rgb(193, 193, 193);")
        self.pushButton_2.setObjectName("pushButton_2")
        self.pushButton_3 = QtWidgets.QPushButton(parent=Form)
        self.pushButton_3.setGeometry(QtCore.QRect(880, 110, 171, 101))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(18)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.pushButton_3.setFont(font)
        self.pushButton_3.setStyleSheet("background-color: rgb(193, 193, 193);")
        self.pushButton_3.setObjectName("pushButton_3")
        self.pushButton_4 = QtWidgets.QPushButton(parent=Form)
        self.pushButton_4.setGeometry(QtCore.QRect(500, 230, 171, 101))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(18)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.pushButton_4.setFont(font)
        self.pushButton_4.setStyleSheet("background-color: rgb(193, 193, 193);")
        self.pushButton_4.setObjectName("pushButton_4")
        self.pushButton_5 = QtWidgets.QPushButton(parent=Form)
        self.pushButton_5.setGeometry(QtCore.QRect(690, 230, 171, 101))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(18)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.pushButton_5.setFont(font)
        self.pushButton_5.setStyleSheet("background-color: rgb(193, 193, 193);")
        self.pushButton_5.setObjectName("pushButton_5")
        self.pushButton_6 = QtWidgets.QPushButton(parent=Form)
        self.pushButton_6.setGeometry(QtCore.QRect(880, 230, 171, 101))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(18)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.pushButton_6.setFont(font)
        self.pushButton_6.setStyleSheet("background-color: rgb(193, 193, 193);")
        self.pushButton_6.setObjectName("pushButton_6")
        self.line = QtWidgets.QFrame(parent=Form)
        self.line.setGeometry(QtCore.QRect(471, 100, 16, 771))
        self.line.setFrameShape(QtWidgets.QFrame.Shape.VLine)
        self.line.setFrameShadow(QtWidgets.QFrame.Shadow.Sunken)
        self.line.setObjectName("line")
        self.label_4 = QtWidgets.QLabel(parent=Form)
        self.label_4.setGeometry(QtCore.QRect(10, 110, 201, 31))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(16)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.label_4.setFont(font)
        self.label_4.setObjectName("label_4")
        self.line_2 = QtWidgets.QFrame(parent=Form)
        self.line_2.setGeometry(QtCore.QRect(0, 90, 1091, 16))
        self.line_2.setFrameShape(QtWidgets.QFrame.Shape.HLine)
        self.line_2.setFrameShadow(QtWidgets.QFrame.Shadow.Sunken)
        self.line_2.setObjectName("line_2")
        self.label_5 = QtWidgets.QLabel(parent=Form)
        self.label_5.setGeometry(QtCore.QRect(10, 60, 71, 31))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(12)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.label_5.setFont(font)
        self.label_5.setObjectName("label_5")
        self.tableWidget = QtWidgets.QTableWidget(parent=Form)
        self.tableWidget.setGeometry(QtCore.QRect(10, 150, 451, 721))
        self.tableWidget.setObjectName("tableWidget")
        self.tableWidget.setColumnCount(0)
        self.tableWidget.setRowCount(0)
        self.label_6 = QtWidgets.QLabel(parent=Form)
        self.label_6.setGeometry(QtCore.QRect(170, 10, 131, 31))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(16)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.label_6.setFont(font)
        self.label_6.setStyleSheet("background-color: rgb(226, 226, 226);")
        self.label_6.setText("")
        self.label_6.setObjectName("label_6")
        self.label_7 = QtWidgets.QLabel(parent=Form)
        self.label_7.setGeometry(QtCore.QRect(100, 60, 571, 31))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(16)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.label_7.setFont(font)
        self.label_7.setStyleSheet("background-color: rgb(226, 226, 226);")
        self.label_7.setText("")
        self.label_7.setObjectName("label_7")
        self.label_8 = QtWidgets.QLabel(parent=Form)
        self.label_8.setGeometry(QtCore.QRect(230, 110, 131, 31))
        font = QtGui.QFont()
        font.setFamily("Palatino Linotype")
        font.setPointSize(14)
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.label_8.setFont(font)
        self.label_8.setStyleSheet("background-color: rgb(226, 226, 226);")
        self.label_8.setText("")
        self.label_8.setObjectName("label_8")
        self.line.raise_()
        self.label_2.raise_()
        self.label.raise_()
        self.label_3.raise_()
        self.pushButton.raise_()
        self.pushButton_2.raise_()
        self.pushButton_3.raise_()
        self.pushButton_4.raise_()
        self.pushButton_5.raise_()
        self.pushButton_6.raise_()
        self.label_4.raise_()
        self.line_2.raise_()
        self.label_5.raise_()
        self.tableWidget.raise_()
        self.label_7.raise_()
        self.label_6.raise_()
        self.label_8.raise_()

        self.retranslateUi(Form)
        QtCore.QMetaObject.connectSlotsByName(Form)

    def retranslateUi(self, Form):
        _translate = QtCore.QCoreApplication.translate
        Form.setWindowTitle(_translate("Form", "Оформление заказа"))
        self.label.setText(_translate("Form", "------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------"))
        self.label_2.setText(_translate("Form", "------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------"))
        self.label_3.setText(_translate("Form", "Стол №"))
        self.pushButton.setText(_translate("Form", "завтрак"))
        self.pushButton_2.setText(_translate("Form", "холодное"))
        self.pushButton_3.setText(_translate("Form", "горячее"))
        self.pushButton_4.setText(_translate("Form", "суп"))
        self.pushButton_5.setText(_translate("Form", "выпечка"))
        self.pushButton_6.setText(_translate("Form", "десерт"))
        self.label_4.setText(_translate("Form", "Состав заказа:"))
        self.label_5.setText(_translate("Form", "кассир:"))
        

#class Zakaz(QMainWindow):
class Zakaz(QWidget):                                      # +++ QWidget
    def __init__(self, parent=None):                       # 2 +++ parent
        super(Zakaz, self).__init__()
        
        self.parent = parent                               # 3 +++

        self.ui = Ui_Form3()
        self.ui.setupUi(self)
#        #self.sto = self.st #здесь в теории хочу записать данные из другого класса
        self.sto = self.parent.st                          # 4 +++
        print(f'self.sto ---> {self.parent.st}')

        self.ui.label_6.setText(self.sto)  # 5 <--- сюда писать self.sto


#class Stol(QMainWindow):
class Stol(QWidget):                                       # +++ QWidget
    def __init__(self):
        super().__init__()

        self.ui = Ui_Form2()
        self.ui.setupUi(self)
        self.stol = self.ui.lineEdit
        self.ui.pushButton.clicked.connect(self.btn)


    def btn(self):
        self.st = self.stol.text() #переформотировала введеное число в текст
        print(self.st)
        self.main_window = Zakaz(self)                     # 1 +++ self
        self.main_window.show()
        self.hide()
        

if __name__ == "__main__":
    app = QApplication([])
    window = Stol()
    window.show()
    app.exec()

Спросите если что-то не понятно.