Как получить индекс элемента в listwidget по нажатию кнопки в нём. Pyside6

Я отметил для вас две шибки, которые вы допустили. Спросите если что-то не понятно.

import sys
'''
from PySide6 import QtWidgets
from PySide6.QtCore import (QCoreApplication, QDate, QDateTime, QLocale,
    QMetaObject, QObject, QPoint, QRect,
    QSize, QTime, QUrl, Qt)
from PySide6.QtGui import (QBrush, QColor, QConicalGradient, QCursor,
    QFont, QFontDatabase, QGradient, QIcon,
    QImage, QKeySequence, QLinearGradient, QPainter,
    QPalette, QPixmap, QRadialGradient)
from PySide6.QtWidgets import *
'''
from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.Qt import *


class Ui_Form(object):
    def setupUi(self, Form):
        if not Form.objectName():
            Form.setObjectName(u"Form")
        Form.resize(291, 53)
        self.horizontalLayout = QHBoxLayout(Form)
        self.horizontalLayout.setObjectName(u"horizontalLayout")
        self.label = QLabel(Form)
        self.label.setObjectName(u"label")
        self.label.setMinimumSize(QSize(200, 20))

        self.horizontalLayout.addWidget(self.label)

        self.pushButton = QPushButton(Form)
        self.pushButton.setObjectName(u"pushButton")
        self.pushButton.setMinimumSize(QSize(50, 20))

        self.horizontalLayout.addWidget(self.pushButton)

        self.retranslateUi(Form)

        QMetaObject.connectSlotsByName(Form)
    # setupUi

    def retranslateUi(self, Form):
        Form.setWindowTitle(QCoreApplication.translate("Form", u"Form", None))
        self.label.setText(QCoreApplication.translate("Form", u"\u0422\u0443\u0442 \u0434\u043e\u043b\u0436\u0435\u043d \u0431\u044b\u0442\u044c \u0442\u0435\u043a\u0441\u0442", None))
        self.pushButton.setText(QCoreApplication.translate("Form", u"PushButton", None))
    # retranslateUi


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        if not MainWindow.objectName():
            MainWindow.setObjectName(u"MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QWidget(MainWindow)
        self.centralwidget.setObjectName(u"centralwidget")
        self.gridLayout = QGridLayout(self.centralwidget)
        self.gridLayout.setObjectName(u"gridLayout")
        self.listWidget = QListWidget(self.centralwidget)
        self.listWidget.setObjectName(u"listWidget")

        self.gridLayout.addWidget(self.listWidget, 0, 0, 1, 1)

        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QMenuBar(MainWindow)
        self.menubar.setObjectName(u"menubar")
        self.menubar.setGeometry(QRect(0, 0, 800, 22))
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QStatusBar(MainWindow)
        self.statusbar.setObjectName(u"statusbar")
        MainWindow.setStatusBar(self.statusbar)

        QMetaObject.connectSlotsByName(MainWindow)


#class TestWidget(QtWidgets.QMainWindow, Ui_Form):
#                          vvvvvvv
class TestWidget(QtWidgets.QWidget, Ui_Form):                                           # +++
    def __init__(self, parent=None):
        super(TestWidget, self).__init__(parent)
        self.setupUi(self)
    # setupUi

    def retranslateUi(self, MainWindow):
        MainWindow.setWindowTitle(QCoreApplication.translate("MainWindow", u"MainWindow", None))
    # retranslateUi

    def setText(self, text):
        self.label.setText(text)

    def sizeHint(self):
        return QSize(200, 40)


class MainWindow(QMainWindow, Ui_MainWindow):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.setupUi(self)
        
        for i in range(20):
            testwidget = TestWidget()
            item = QtWidgets.QListWidgetItem(self.listWidget)
            testwidget.setText(f"Это элемент {i}")
# +++                        
#            testwidget.pushButton.clicked.connect(lambda: self.test(i))                                      
            testwidget.pushButton.clicked.connect(lambda ch, i=i: self.test(i))          # +++
            
            item.setSizeHint(testwidget.sizeHint())
            self.listWidget.addItem(item)
            self.listWidget.setItemWidget(item, testwidget)

    def test(self, i):
        print("Элемент", i)


if __name__ == "__main__":
    if not QtWidgets.QApplication.instance():
        app = QtWidgets.QApplication(sys.argv)
    else:
        app = QtWidgets.QApplication.instance()
    window = MainWindow()
    window.show()
    sys.exit(app.exec())