Как можно сделать возможность нажатия нескольких QRadioButton?

Если вы хотите этого поведения, вам нужно использовать QButtonGroup.

Класс QButtonGroup предоставляет контейнер для организации групп виджетов-кнопок.

import sys
from PyQt5 import QtWidgets, QtGui, QtCore


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(316, 348)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setStyleSheet("background-color: rgb(205, 205, 205)")
        self.centralwidget.setObjectName("centralwidget")
        self.verticalLayout = QtWidgets.QVBoxLayout(self.centralwidget)
        self.verticalLayout.setObjectName("verticalLayout")
        self.label = QtWidgets.QLabel(self.centralwidget)
        font = QtGui.QFont()
        font.setFamily("Sans Serif Collection")
        font.setPointSize(12)
        self.label.setFont(font)
        self.label.setStyleSheet("text-align: center;")
        self.label.setAlignment(QtCore.Qt.AlignCenter)
        self.label.setObjectName("label")
        self.verticalLayout.addWidget(self.label)
        self.label_2 = QtWidgets.QLabel(self.centralwidget)
        self.label_2.setObjectName("label_2")
        self.verticalLayout.addWidget(self.label_2)
        self.horizontalLayout = QtWidgets.QHBoxLayout()
        self.horizontalLayout.setObjectName("horizontalLayout")
        self.radioButton = QtWidgets.QRadioButton(self.centralwidget)
        self.radioButton.setObjectName("radioButton")
        self.horizontalLayout.addWidget(self.radioButton)
        self.radioButton_3 = QtWidgets.QRadioButton(self.centralwidget)
        self.radioButton_3.setObjectName("radioButton_3")
        self.horizontalLayout.addWidget(self.radioButton_3)
        self.radioButton_2 = QtWidgets.QRadioButton(self.centralwidget)
        self.radioButton_2.setObjectName("radioButton_2")
        self.horizontalLayout.addWidget(self.radioButton_2)
        self.verticalLayout.addLayout(self.horizontalLayout)
        self.label_3 = QtWidgets.QLabel(self.centralwidget)
        self.label_3.setObjectName("label_3")
        self.verticalLayout.addWidget(self.label_3)
        self.horizontalLayout_2 = QtWidgets.QHBoxLayout()
        self.horizontalLayout_2.setObjectName("horizontalLayout_2")
        self.radioButton_4 = QtWidgets.QRadioButton(self.centralwidget)
        self.radioButton_4.setObjectName("radioButton_4")
        self.horizontalLayout_2.addWidget(self.radioButton_4)
        self.radioButton_5 = QtWidgets.QRadioButton(self.centralwidget)
        self.radioButton_5.setObjectName("radioButton_5")
        self.horizontalLayout_2.addWidget(self.radioButton_5)
        self.radioButton_6 = QtWidgets.QRadioButton(self.centralwidget)
        self.radioButton_6.setObjectName("radioButton_6")
        self.horizontalLayout_2.addWidget(self.radioButton_6)
        self.verticalLayout.addLayout(self.horizontalLayout_2)
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setObjectName("pushButton")
        self.verticalLayout.addWidget(self.pushButton)
        self.label_5 = QtWidgets.QLabel(self.centralwidget)
        self.label_5.setObjectName("label_5")
        self.verticalLayout.addWidget(self.label_5)
        self.label_4 = QtWidgets.QLabel(self.centralwidget)
        font = QtGui.QFont()
        font.setFamily("Segoe UI Variable Small Semibol")
        font.setPointSize(28)
        font.setBold(True)
        font.setWeight(75)
        self.label_4.setFont(font)
        self.label_4.setStyleSheet("background-color: rgb(255, 255, 255)")
        self.label_4.setText("")
        self.label_4.setAlignment(QtCore.Qt.AlignCenter)
        self.label_4.setObjectName("label_4")
        self.verticalLayout.addWidget(self.label_4)
        self.horizontalLayout_3 = QtWidgets.QHBoxLayout()
        self.horizontalLayout_3.setObjectName("horizontalLayout_3")
        self.label_9 = QtWidgets.QLabel(self.centralwidget)
        self.label_9.setObjectName("label_9")
        self.horizontalLayout_3.addWidget(self.label_9)
        self.label_10 = QtWidgets.QLabel(self.centralwidget)
        self.label_10.setObjectName("label_10")
        self.horizontalLayout_3.addWidget(self.label_10)
        self.label_11 = QtWidgets.QLabel(self.centralwidget)
        self.label_11.setObjectName("label_11")
        self.horizontalLayout_3.addWidget(self.label_11)
        self.verticalLayout.addLayout(self.horizontalLayout_3)
        self.horizontalLayout_4 = QtWidgets.QHBoxLayout()
        self.horizontalLayout_4.setObjectName("horizontalLayout_4")
        self.label_6 = QtWidgets.QLabel(self.centralwidget)
        self.label_6.setStyleSheet("background-color: rgb(255, 255, 255)")
        self.label_6.setText("")
        self.label_6.setAlignment(QtCore.Qt.AlignCenter)
        self.label_6.setObjectName("label_6")
        self.horizontalLayout_4.addWidget(self.label_6)
        self.label_7 = QtWidgets.QLabel(self.centralwidget)
        self.label_7.setStyleSheet("background-color: rgb(255, 255, 255);")
        self.label_7.setText("")
        self.label_7.setAlignment(QtCore.Qt.AlignCenter)
        self.label_7.setObjectName("label_7")
        self.horizontalLayout_4.addWidget(self.label_7)
        self.label_8 = QtWidgets.QLabel(self.centralwidget)
        self.label_8.setStyleSheet("background-color: rgb(255, 255, 255);")
        self.label_8.setText("")
        self.label_8.setAlignment(QtCore.Qt.AlignCenter)
        self.label_8.setObjectName("label_8")
        self.horizontalLayout_4.addWidget(self.label_8)
        self.verticalLayout.addLayout(self.horizontalLayout_4)
        self.pushButton_2 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_2.setObjectName("pushButton_2")
        self.verticalLayout.addWidget(self.pushButton_2)
        MainWindow.setCentralWidget(self.centralwidget)

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

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.label.setText(_translate("MainWindow", "КАЛЬКУЛЯТОР БЖУ"))
        self.label_2.setText(_translate("MainWindow", "Выбери активность в течение дня:"))
        self.radioButton.setText(_translate("MainWindow", "Спорт"))
        self.radioButton_3.setText(_translate("MainWindow", "Отдых"))
        self.radioButton_2.setText(_translate("MainWindow", "Норма"))
        self.label_3.setText(_translate("MainWindow", "Выбери цель дня:"))
        self.radioButton_4.setText(_translate("MainWindow", "Похудение"))
        self.radioButton_5.setText(_translate("MainWindow", "Набор веса"))
        self.radioButton_6.setText(_translate("MainWindow", "Поддержание массы"))
        self.pushButton.setText(_translate("MainWindow", "РАССЧИТАТЬ"))
        self.label_5.setText(_translate("MainWindow", "Ваша номра ККАЛ сегодня:"))
        self.label_9.setText(_translate("MainWindow", "Белки:"))
        self.label_10.setText(_translate("MainWindow", "Углеводы:"))
        self.label_11.setText(_translate("MainWindow", "Жиры:"))
        self.pushButton_2.setText(_translate("MainWindow", "МЕНЮ"))
        

class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()
        self.setupUi(self) 
   
        group_button = QtWidgets.QButtonGroup(self)            # <---
        group_button.addButton(self.radioButton)               # <---        
        group_button.addButton(self.radioButton_2)             # <---         
        group_button.addButton(self.radioButton_3)             # <---         
        

if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.resize(410, 348)
    w.show()
    sys.exit(app.exec())