Sudoku Verifications blocking any move attempt

Question

So I'm trying to make a sudoku game and it's coming along fairly well except at the state I'm at the program gives me an error for every move I try to make. isRow checks if a move is possible on the row and so on and isPossible combines the 3 to check if a move is possible or not. Can anyone tell me what is wrong with my verifications?

board = list(range(81)) # Creates a board with 9*9 elements

board2 = board # Duplicates this board for later verfications

def showLine(start, end): # Prints each line of the sudoku puzzle
    for i in range(start, end):
        if i < end-1:
            print(str(board[i]), '|', end =" ")
        else:
            print(str(board[i]))

def showGroup(start): # Prints each 9*3 group of regions (1/3 of the puzzle)
    for i in range(3):
        end = start + 9
        showLine(start, end)
        start += 9
    if start != 81:
        print('----------+-----------+-----------')

def show(): # Prints whole puzzle
    for i in range(0, 55, 27):
        showGroup(i)

rows = []
columns = []
groups = []

for i in range(0, 80, 9): # Cretes a list of rows
    rows.append(board[i:i+9])

for i in range(9): # Creates a list of columns
    columns.append(board[i::9])

temp = 0

for x in range(0, 61, 3): # Creates a list of groups (3*3 regions)
    group = []
    for i in range(3):
        for j in range(3):
            temp = x + 9*i + j
            group.append(temp)
    groups.append(group)

#Duplicating rows columns and groups for verifications
rows2 = rows
columns2 = columns
groups2 = groups

def isRow(cell, number): # Checking if an introduces number isn't repeating in the row
    x = 0
    for row in rows2:
        if cell in row:
            x = rows2.index(row)
        if number in rows[x]:
            return False
        else:
            return True

def isColumn(cell, number):): # Checking if an introduces number isn't repeating in the column
    x = 0
    for column in columns2:
        if cell in column:
            x = columns2.index(column)
        if number in columns[x]:
            return False
        else:
            return True

def isGroup(cell, number):): # Checking if an introduces number isn't repeating in the group
    x = 0
    for group in groups2:
        if cell in group:
            x = groups2.index(group)
        if number in groups[x]:
            return False
        else:
            return True

def isPossible(cell, number): # Combines the last 3 verifications
    if isRow(cell, number) and isColumn(cell, number) and isGroup(cell, number):
        return True
    else:
        return False

for i in board: # Fills the board with 0's to not interfere with the game
    board[i] = 0

while True:
    show()


    selected_cell = int(input("Please select an available cell: "))
    number = int(input("Please select a number for that cell: "))

    if isPossible(selected_cell, number):
        board[selected_cell] = number
    else:
        print("Error, please try again")