Why is this n-queens recursive solution not working?

Question

here is the code:
from copy import deepcopy as dc
class Solution:
def solveNQueens(self, n: int) -> List[List[str]]:
result = list()
board =[["."]*n]*n

def issafe(row, col, board):
#check the left elements in row
for i in range(col+1):
if board[row][i]=="Q":
return False

row-=1
col-=1
row2 = row
col2 = col

#check down diagonal elements
while row>=0 and col>=0:
if board[row][col]=="Q":
return False
row-=1
col-=1

while col2>=0 and row2<n:
if board[row2][col2]=="Q":
return False
row2+=1
col2-=1

return True

def helper(col, board):
if col==n:
result.append(["".join(row) for row in board])
return

for row in range(n):
if issafe(row, col, dc(board)):
board[row][col] = "Q"
helper(col+1, dc(board))
board[row][col] = "."

helper(0, dc(board))
return result

Answer 1

Put the code in Code Playground and include a link to it in the question description. This allows us to see the problem and test solutions.
Also, explain what didn't work. Good communication is key to get good help.