Login window, password verification

Try this way:

import sys
from PyQt5 import QtCore, QtGui, QtWidgets

#from design2 import Ui_MainWindow

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(400, 600)
        MainWindow.setMinimumSize(QtCore.QSize(400, 600))
        MainWindow.setMaximumSize(QtCore.QSize(400, 600))
        font = QtGui.QFont()
        font.setFamily("HelveticaNeueCyr")
        font.setPointSize(8)
        font.setStyleStrategy(QtGui.QFont.PreferAntialias)
        MainWindow.setFont(font)
        MainWindow.setWindowOpacity(1.0)
        MainWindow.setStyleSheet("background-color: qlineargradient(spread:pad, x1:0.011, y1:1, x2:1, y2:0, stop:0 rgba(182, 29, 212, 255), stop:1 rgba(48, 7, 182, 255));")
        MainWindow.setTabShape(QtWidgets.QTabWidget.Rounded)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(100, 400, 201, 71))
        font = QtGui.QFont()
        font.setFamily("HelveticaNeueCyr")
        font.setPointSize(22)
        self.pushButton.setFont(font)
        self.pushButton.setCursor(QtGui.QCursor(QtCore.Qt.OpenHandCursor))
        self.pushButton.setStyleSheet("color: rgb(255, 255, 255);\n"
"border-radius: 30px;\n"
"background-color: qlineargradient(spread:repeat, x1:0, y1:0.545, x2:1, y2:0.563, stop:0 rgba(53, 255, 255, 255), stop:1 rgba(42, 255, 177, 255));")
        self.pushButton.setObjectName("pushButton")
        self.lineEdit = QtWidgets.QLineEdit(self.centralwidget)
        self.lineEdit.setGeometry(QtCore.QRect(70, 200, 261, 61))
        font = QtGui.QFont()
        font.setFamily("HelveticaNeueCyr")
        font.setPointSize(22)
        self.lineEdit.setFont(font)
        self.lineEdit.setStyleSheet("background-color: rgb(255, 255, 255);\n"
"border-radius: 30px;")
        self.lineEdit.setMaxLength(100)
        self.lineEdit.setAlignment(QtCore.Qt.AlignCenter)
        self.lineEdit.setClearButtonEnabled(True)
        self.lineEdit.setObjectName("lineEdit")
        self.lineEdit_2 = QtWidgets.QLineEdit(self.centralwidget)
        self.lineEdit_2.setGeometry(QtCore.QRect(70, 290, 261, 61))
        font = QtGui.QFont()
        font.setFamily("HelveticaNeueCyr")
        font.setPointSize(22)
        font.setItalic(False)
        font.setUnderline(False)
        font.setStrikeOut(False)
        font.setStyleStrategy(QtGui.QFont.PreferAntialias)
        self.lineEdit_2.setFont(font)
        self.lineEdit_2.setAutoFillBackground(False)
        self.lineEdit_2.setStyleSheet("background-color: rgb(255, 255, 255);\n"
"border-radius: 30px;\n"
"\n"
"")
        self.lineEdit_2.setMaxLength(12)
        self.lineEdit_2.setEchoMode(QtWidgets.QLineEdit.Password)
        self.lineEdit_2.setCursorPosition(0)
        self.lineEdit_2.setAlignment(QtCore.Qt.AlignCenter)
        self.lineEdit_2.setDragEnabled(False)
        self.lineEdit_2.setReadOnly(False)
        self.lineEdit_2.setClearButtonEnabled(True)
        self.lineEdit_2.setObjectName("lineEdit_2")
        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(120, 90, 181, 101))
        font = QtGui.QFont()
        font.setFamily("HelveticaNeueCyr")
        font.setPointSize(35)
        self.label.setFont(font)
        self.label.setStyleSheet("background-color: rgb(alpha 0.5);\n"
"color: rgb(255, 255, 255);")
        self.label.setObjectName("label")
        self.label_2 = QtWidgets.QLabel(self.centralwidget)
        self.label_2.setGeometry(QtCore.QRect(110, 360, 201, 16))
        font = QtGui.QFont()
        font.setFamily("HelveticaNeueCyr")
        font.setPointSize(10)
        font.setStyleStrategy(QtGui.QFont.PreferAntialias)
        self.label_2.setFont(font)
        self.label_2.setStyleSheet("background-color: rgb(alpha 0.5);\n"
"color: rgb(255, 255, 255);")
        self.label_2.setObjectName("label_2")
        self.label_3 = QtWidgets.QLabel(self.centralwidget)
        self.label_3.setGeometry(QtCore.QRect(100, 360, 16, 16))
        font = QtGui.QFont()
        font.setPointSize(11)
        self.label_3.setFont(font)
        self.label_3.setStyleSheet("background-color: rgb(alpha 0.5);\n"
"color: rgb(255, 255, 255);")
        self.label_3.setObjectName("label_3")
        MainWindow.setCentralWidget(self.centralwidget)

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

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "Вход"))
        self.pushButton.setText(_translate("MainWindow", "Войти"))
        self.lineEdit.setPlaceholderText(_translate("MainWindow", "Введите логин"))
        self.lineEdit_2.setPlaceholderText(_translate("MainWindow", "Введите пароль"))
        self.label.setText(_translate("MainWindow", "Войдите"))
        self.label_2.setText(_translate("MainWindow", "Максимальная длина пароля - 12"))
        self.label_3.setText(_translate("MainWindow", "*"))


class ExampleApp(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()
        self.setupUi(self)

        self.pushButton.clicked.connect(self.onClicked)                        # +++

    def onClicked(self):                                                       # +++
        if self.lineEdit.text() != '123' or self.lineEdit_2.text() != '1234':
            msg = QtWidgets.QMessageBox.information(self, 'Error', 'Неправильный логин или пароль')
        else:
            msg = QtWidgets.QMessageBox.information(self, 'Welcom', 'Добро пожаловать!')


if __name__=="__main__":
    app = QtWidgets.QApplication(sys.argv)
    window = ExampleApp()
    window.show()
    sys.exit(app.exec_())