What wrong with this? Thank you so much!

def checkio(field):
    rotated_field = zip(*field)
    for row in field:
        if row[0] == row[1] == row[2] and row[0]!= ".":
            return row[0]
        else:
            for col in rotated_field:
                if col[0] == col[1] == col[2] and col[0]!=".":
                    return col[0]
                else:
                    if field[0][0] == field[1][1] == field [2][2] and field[0][0]!=".":
                        return field[0][0]
                    else:
                        if field[0][2] == field[1][1] == field[2][0] and field[0][2]!=".":
                            return field[0][2]
                        else:
                            return "D"