Создал GUI в QT Designer. Пытаюсь вставить фнкционал/формулу рассчета индекса массы тела. При запуске программа crashing

У вас все сигналы привязаны к одному слоту. Чтобы не было ошибки, надо вставить проверку делителя на ноль.

Не изменяйте код, сгенерированный Qt Designer. Создайте другой класс, который наследуется от соответствующего виджета, и используйте созданный класс для его заполнения.

from PyQt5 import QtCore, QtGui, QtWidgets


class Ui_Form(object):
    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(317, 300)
        Form.setAutoFillBackground(False)
        Form.setStyleSheet("background-color: rgb(211, 244, 255);\n"
                           "border-color: rgb(43, 43, 43);")
        self.pushButton = QtWidgets.QPushButton(Form)
        self.pushButton.setGeometry(QtCore.QRect(200, 80, 91, 51))
        font = QtGui.QFont()
        font.setFamily("Arial")
        font.setPointSize(10)
        font.setBold(True)
        font.setWeight(75)
        self.pushButton.setFont(font)
        self.pushButton.setCursor(QtGui.QCursor(QtCore.Qt.PointingHandCursor))
        self.pushButton.setStyleSheet("QPushButton {\n"
                                      "    background-color: rgb(0, 85, 127);\n"
                                      "    color: white\n"
                                      "}\n"
                                      "    ")
        self.pushButton.setObjectName("pushButton")
        self.label = QtWidgets.QLabel(Form)
        self.label.setGeometry(QtCore.QRect(30, 80, 111, 21))
        self.label.setObjectName("label")
        self.label_2 = QtWidgets.QLabel(Form)
        self.label_2.setGeometry(QtCore.QRect(30, 110, 111, 21))
        self.label_2.setObjectName("label_2")
        self.label_3 = QtWidgets.QLabel(Form)
        self.label_3.setGeometry(QtCore.QRect(30, 10, 261, 16))
        font = QtGui.QFont()
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.label_3.setFont(font)
        self.label_3.setObjectName("label_3")
        self.label_4 = QtWidgets.QLabel(Form)
        self.label_4.setGeometry(QtCore.QRect(70, 30, 201, 16))
        font = QtGui.QFont()
        font.setBold(True)
        font.setItalic(True)
        font.setWeight(75)
        self.label_4.setFont(font)
        self.label_4.setObjectName("label_4")
        self.label_5 = QtWidgets.QLabel(Form)
        self.label_5.setGeometry(QtCore.QRect(90, 170, 61, 20))
        self.label_5.setObjectName("label_5")
        self.lineEdit_3 = QtWidgets.QLineEdit(Form)
        self.lineEdit_3.setGeometry(QtCore.QRect(30, 240, 261, 41))
        self.lineEdit_3.setStyleSheet("background-color: rgb(255, 255, 255);")
        self.lineEdit_3.setReadOnly(True)
        self.lineEdit_3.setObjectName("lineEdit_3")
        self.label_6 = QtWidgets.QLabel(Form)
        self.label_6.setGeometry(QtCore.QRect(100, 210, 121, 16))
        font = QtGui.QFont()
        font.setFamily("Arial Black")
        font.setPointSize(14)
        font.setBold(True)
        font.setWeight(75)
        self.label_6.setFont(font)
        self.label_6.setCursor(QtGui.QCursor(QtCore.Qt.CrossCursor))
        self.label_6.setObjectName("label_6")
        self.lineEdit_4 = QtWidgets.QLineEdit(Form)
        self.lineEdit_4.setGeometry(QtCore.QRect(150, 160, 141, 31))
        font = QtGui.QFont()
        font.setFamily("Arial")
        font.setPointSize(14)
        font.setBold(True)
        font.setWeight(75)
        self.lineEdit_4.setFont(font)
        self.lineEdit_4.setStyleSheet("background-color: rgb(47, 61, 255);\n"
                                      "color: rgb(255, 255, 255);")
        self.lineEdit_4.setObjectName("lineEdit_4")
        self.spinBox = QtWidgets.QSpinBox(Form)
        self.spinBox.setGeometry(QtCore.QRect(140, 80, 51, 22))
        self.spinBox.setMaximum(999)
        self.spinBox.setObjectName("spinBox")
#        self.spinBox.valueChanged.connect(self.result)
        self.spinBox_2 = QtWidgets.QSpinBox(Form)
        self.spinBox_2.setGeometry(QtCore.QRect(140, 110, 51, 22))
        self.spinBox_2.setMaximum(999)
        self.spinBox_2.setObjectName("spinBox_2")
#        self.spinBox_2.valueChanged.connect(self.result)

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

    def retranslateUi(self, Form):
        _translate = QtCore.QCoreApplication.translate
        Form.setWindowTitle(_translate("Form", "BMI Calculator"))
        self.pushButton.setText(_translate("Form", "Calculate"))
#        self.pushButton.clicked.connect(self.result)
        self.label.setText(_translate("Form", "Enter your WEIGTH"))
        self.label_2.setText(_translate("Form", "Enter your HEIGTH"))
        self.label_3.setText(_translate("Form", "To calculate your BMI please enter your data"))
        self.label_4.setText(_translate("Form", "below and press Calculate button"))
        self.label_5.setText(_translate("Form", "Your BMI is"))
        self.label_6.setText(_translate("Form", "Conclusion"))

#    def result(self):
#        self.lineEdit_4.setText(self.spinBox.value() / self.spinBox_2.value()) ** 2


class MenuWindow(QtWidgets.QWidget, Ui_Form):
    def __init__(self):
        super().__init__()
        self.setupUi(self)

        self.spinBox.valueChanged.connect(self.result)
        self.spinBox_2.valueChanged.connect(self.result)
        self.pushButton.clicked.connect(self.result)

    def result(self):
#        self.lineEdit_4.setText(self.spinBox.value() / self.spinBox_2.value()) ** 2

        sb_2 = self.spinBox_2.value()
        if sb_2:
            self.lineEdit_4.setText(f'{(self.spinBox.value() / sb_2) ** 2 : .3f}')
        else:
            self.lineEdit_4.clear()
        

if __name__ == "__main__":
    import sys

    app = QtWidgets.QApplication(sys.argv)
#    Form = QtWidgets.QWidget()
#    ui = Ui_Form()
#    ui.setupUi(Form)
#    Form.show()
    w = MenuWindow()
    w.show()
    sys.exit(app.exec_())