NameError: name 'n' is not defined but i dont know what to do(new to coding)

i added this to my minesweeper game so i could choose different difficulties yet when i run it it returns:
NameError: name 'n' is not defined

def Game():
        
        difficulty = input("Select your difficulty (b, i, h):")
        if difficulty.lower() == 'b':
            n = 5
            mines_no = 5
        elif difficulty.lower() == 'i':
            n = 6
            mines_no = 10
        else:
            n = 8
            mines_no = 20 
 
if __name__ == "__main__":
    
    Game()
 
    # The actual values of the grid
    numbers = [[0 for y in range(n)] for x in range(n)] 
    # The apparent values of the grid
    mine_values = [[' ' for y in range(n)] for x in range(n)]

so i expected after i ran set code that the game would just start because before it did start(when i didn't have the difficulty select screen yet)
instead i had

if __name__ == "__main__":
    
    n = 5
    mines_no = 5
 
    # The actual values of the grid
    numbers = [[0 for y in range(n)] for x in range(n)] 
    # The apparent values of the grid
    mine_values = [[' ' for y in range(n)] for x in range(n)]

You'll need to read up on the differences between local and global variables; once you wrap things into def Game():, you're writing to local n and mines_no and you'd need to e.g. return them.

Once you do that, it's better to wrap your entire game in a function, so you're not mixing and matching globals and locals (it's good coding hygiene). I also took the liberty of just returning the two values as a tuple from get_difficulty, and unpacking them in the new main game() function.

def get_difficulty():
    difficulty = input("Select your difficulty (b, i, h):")
    if difficulty.lower() == "b":
        return (5, 5)
    elif difficulty.lower() == "i":
        return (6, 10)
    else:
        return (8, 20)


def game():

    n, mines_no = get_difficulty()

    # The actual values of the grid
    numbers = [[0 for y in range(n)] for x in range(n)]
    # The apparent values of the grid
    mine_values = [[" " for y in range(n)] for x in range(n)]


if __name__ == "__main__":
    game()

The variable n is inside the Game function.
If you return the values, you can use them:

def Game():
    difficulty = input("Select your difficulty (b, i, h):")
    if difficulty.lower() == 'b':
        n = 5
        mines_no = 5
    elif difficulty.lower() == 'i':
        n = 6
        mines_no = 10
    else:
        n = 8
        mines_no = 20 
    return n, mines_no
 
if __name__ == "__main__":
    
    n, mines_no = Game()