Python detects a variable as local, even if it is set prior to definition

I’ve been trying to do an assignment for my python exam and I got stuck on this problem. I’ve listed the full code bellow. I set the variable A1 to " " and later down the line I want to use the function player1_move to change the variables accordingly to the user’s input. However I get errors saying that A1 is mentioned before being set. I think that python recognizes A1 as a local variable within the definition (sorry if I’m saying gibberish but I can’t describe it more accurately).
How can I fix this error?
Code:

A1 = ""
A2 = " "
A3 = " "
B1 = " "
B2 = " "
B3 = " "
C1 = " "
C2 = " "
C3 = " "

def board():
    print(f"""  1   2   3 
    A {A1}---{A2}---{A3}
      |  |  /|
      |  | / |
      |  |/  |
     B {B1}---{B2}---{B3}
      |  /|  |
      | / |  |
      |/  |  |
     G {C1}---{C2}---{C3}""")

board()
def player1_move(x):

    if x.title() == "A1":
        if A1 == " ":
            A1 = "x"
            board()
        else:
            print("This place is taken!" + "nEnter another one!")

player1move1 = input("Player 1 moves first!" + "nEnter your first move! :")
player1_move(player1move1)

Error:

Traceback (most recent call last):
  File "/home/noname/Documents/PythonProjects/Assignment/dummy_file1.py", line 34, in <module>
    player1_move(player1move1)
  File "/home/noname/Documents/PythonProjects/Assignment/dummy_file1.py", line 27, in player1_move
    if A1 == " ":
UnboundLocalError: local variable 'A1' referenced before assignment