Почему не запускается PyQt5 код в PyCharm?

Я отметил для вас изменения, которые надо сделать. Если что-то не понятно, спросите.

import sys
# ??? import time
from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtWidgets import QApplication, QMainWindow
from PyQt5.QtCore import QDateTime, QDate, QTime, Qt


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(649, 168)
        MainWindow.setMinimumSize(QtCore.QSize(0, 0))
        MainWindow.setMaximumSize(QtCore.QSize(999999, 999999))
        MainWindow.setStyleSheet("MainWindow\n""{\n""    background-color: rgb(37, 37, 37);\n""}")
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setMaximumSize(QtCore.QSize(99999, 999999))
        self.centralwidget.setStyleSheet("background-color: rgb(37, 37, 37)\n""")
        self.centralwidget.setObjectName("centralwidget")
        self.frame = QtWidgets.QFrame(self.centralwidget)
        self.frame.setGeometry(QtCore.QRect(0, -1, 641, 171))
        self.frame.setMinimumSize(QtCore.QSize(0, 0))
        self.frame.setMaximumSize(QtCore.QSize(99999, 999))
        self.frame.setStyleSheet("MainWindow\n""{\n""    background-color:rgb(46, 46, 46)\n""}")
        self.frame.setFrameShape(QtWidgets.QFrame.NoFrame)
        self.frame.setFrameShadow(QtWidgets.QFrame.Raised)
        self.frame.setObjectName("frame")
        self.tabWidget = QtWidgets.QTabWidget(self.frame)
        self.tabWidget.setGeometry(QtCore.QRect(0, 0, 651, 161))
        self.tabWidget.setMinimumSize(QtCore.QSize(4, 0))
        font = QtGui.QFont()
        font.setFamily("Rio Glamour personal use")
        font.setPointSize(14)
        self.tabWidget.setFont(font)
        self.tabWidget.setStyleSheet("QTabWidget::pane\n""{\n""    border: 1px;\n""    background: rgb(37, 37, 37);\n""}\n""\n""QTabBar::tab\n""{\n""    background: rgb(37, 37, 37);\n""    color:rgb(255, 255, 255);\n""}\n""\n""\n""QTabBar::tab:selected\n""{\n""    background:rgb(26, 26, 26);\n""    color:rgb(255, 255, 255);\n""}\n""\n""QTabBar::tab:hover\n""{\n""    background:rgb(57, 57, 57);\n""    color:rgb(255, 255, 255);\n""}\n""\n""")
        self.tabWidget.setObjectName("tabWidget")
        self.tab = QtWidgets.QWidget()
        font = QtGui.QFont()
        font.setPointSize(8)
        self.tab.setFont(font)
        self.tab.setObjectName("tab")
        self.label = QtWidgets.QLabel(self.tab)
        self.label.setGeometry(QtCore.QRect(-4, -8, 651, 141))
        self.label.setText("")
        self.label.setObjectName("label")
        self.tabWidget.addTab(self.tab, "")
        self.tab_2 = QtWidgets.QWidget()
        self.tab_2.setObjectName("tab_2")
        self.label_2 = QtWidgets.QLabel(self.tab_2)
        self.label_2.setGeometry(QtCore.QRect(0, 0, 661, 141))
        self.label_2.setText("")
        self.label_2.setObjectName("label_2")
        self.tabWidget.addTab(self.tab_2, "")
        self.tab_3 = QtWidgets.QWidget()
        self.tab_3.setStyleSheet("background-color: rgb(37, 37, 37);\n""color: rgb(255, 255, 255);")
        self.tab_3.setObjectName("tab_3")
        self.label_3 = QtWidgets.QLabel(self.tab_3)
        self.label_3.setGeometry(QtCore.QRect(10, 0, 661, 141))
        self.label_3.setText("")
        self.label_3.setObjectName("label_3")
        self.tabWidget.addTab(self.tab_3, "")
        MainWindow.setCentralWidget(self.centralwidget)

        self.retranslateUi(MainWindow)
        self.tabWidget.setCurrentIndex(2)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab), _translate("MainWindow", "Time"))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab_2), _translate("MainWindow", "Stopwatch"))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab_3), _translate("MainWindow", "Timer"))
        
    
class ManinWindow(QtWidgets.QMainWindow, Ui_MainWindow):                  # !!! ManinWindow
    def __init__(self):
        super().__init__()
# -----------------> vvvv  
        self.setupUi(self)

# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv        
        self.tabWidget.currentChanged.connect(self.current_index)
        self.tabWidget.setCurrentIndex(0)
        
    def add_functions(self):
# --------------------------------------------------> vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
        self.label.setText(QDateTime.currentDateTime().toString('yyyy-MM-dd HH:mm:ss')) 
        self.label.setFont(QtGui.QFont("Rio Glamour personal use", 40))
        self.label.setStyleSheet("color: #DADDFC;")                    # +++

    def current_index(self, index):                                    # +++
        if index == 0:
            self.add_functions()
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^    
    

if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
# ----> vvvvvvvvvvvvv
    w = ManinWindow()                                                      # !!! ManinWindow
    w.show()
    sys.exit(app.exec_())