Как убрать рамку главного окна QMainWindow в PyQt6?

Правильно так:

# from PyQt6 import QtCore, QtWidgets                                 # PyQt6
from PyQt5 import QtCore, QtWidgets                                   # PyQt5


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(371, 323)
        self.centralwidget = QtWidgets.QWidget(parent=MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.frame = QtWidgets.QFrame(parent=self.centralwidget)
        self.frame.setGeometry(QtCore.QRect(20, 10, 341, 271))
        self.frame.setStyleSheet("QFrame {\n"
"    border-radius: 10px;\n"
"    background-color: rgb(68,56,72);\n"
"}")
        self.frame.setFrameShape(QtWidgets.QFrame.Shape.StyledPanel)
        self.frame.setFrameShadow(QtWidgets.QFrame.Shadow.Raised)
        self.frame.setObjectName("frame")
        self.calendarWidget = QtWidgets.QCalendarWidget(parent=self.frame)
        self.calendarWidget.setEnabled(True)
        self.calendarWidget.setGeometry(QtCore.QRect(20, 10, 301, 191))
        self.calendarWidget.setToolTip("")
        self.calendarWidget.setStatusTip("")
        self.calendarWidget.setAutoFillBackground(False)
        self.calendarWidget.setStyleSheet("QCalendarWidget{border-radius: 10px;}\n"
"\n"
"\n"
"QCalendarWidget QWidget \n"
"{\n"
"    alternate-background-color: rgb(68,56,72);\n"
"    color:  #FFFFFF;\n"
"    selection-background-color:qlineargradient(spread:pad, x1:1, y1:1, x2:0, y2:0, stop:0 rgba(81, 0, 135, 255), stop:0.427447 rgba(41, 61, 132, 235), stop:1 rgba(155, 79, 165, 255));\n"
"    background-color: rgb(68,56,72);\n"
"    gridline-color: rgb(179, 65, 244);\n"
"    selection-color: #FFFFFF;\n"
"    font: 57 8pt \"Inter Medium\";\n"
"}\n"
"\n"
"\n"
"\n"
"#qt_calendar_prevmonth,\n"
"#qt_calendar_nextmonth{\n"
"border: none;\n"
"qproperty-icon: none;\n"
"min-width: 13px;\n"
"max-width: 13px;\n"
"min-height: 13px;\n"
"max-height: 13px;\n"
"\n"
"border-radius: 10px;\n"
"background-color: transparent;\n"
"padding: 10px;\n"
"\n"
"}\n"
"\n"
"#qt_calendar_prevmonth{\n"
"margin-right: 5px;\n"
"    image: url(:/images/Vector2.png);\n"
"}\n"
"\n"
"#qt_calendar_nextmonth{\n"
"margin-right: 5px;\n"
"    image: url(:/images/Vector!!.png);\n"
"}\n"
"\n"
"QCalendarWidget QAbstractItemView:disabled \n"
"{ \n"
"color: rgb(64, 64, 64); \n"
"}\n"
"")
        self.calendarWidget.setGridVisible(False)
        self.calendarWidget.setNavigationBarVisible(True)
        self.calendarWidget.setDateEditEnabled(True)
        self.calendarWidget.setObjectName("calendarWidget")
        self.pushButton = QtWidgets.QPushButton(parent=self.frame)
        self.pushButton.setGeometry(QtCore.QRect(100, 220, 151, 31))
        self.pushButton.setStyleSheet("background:#7C7174;\n"
"border-radius: 10px;\n"
"color: #FFFFFF;\n"
"font: 75 8pt \"MS Shell Dlg 2\";")
        self.pushButton.setObjectName("pushButton")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(parent=MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 371, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(parent=MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)
        
        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)


    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton.setText(_translate("MainWindow", "Выбрать дату отчета"))
        
# - class MainWindow1(Ui_MainWindow):
class MainWindow1(QtWidgets.QMainWindow, Ui_MainWindow):               # !!! +++ 
    def __init__(self):
        super().__init__()
        
        self.setupUi(self)                                             # !!! +++ 
        
#       self.setWindowFlag(QtCore.Qt.WindowType.FramelessWindowHint)   # PyQt6
        self.setWindowFlag(QtCore.Qt.FramelessWindowHint)              # PyQt5
        
        self.setAttribute(QtCore.Qt.WA_TranslucentBackground)       


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
#-    MainWindow1 = QtWidgets.QMainWindow()
#-    ui = Ui_MainWindow()
#-    ui.setupUi(MainWindow1)
#-    MainWindow1.show()

    w = MainWindow1()                                              # !!! +++ 
    w.show()                                                       # !!! +++ 
    
    sys.exit(app.exec())