def login():
  mycursor = mydb.cursor()
  print('Enter username: ')
  username = input()
  checkUsername = mycursor.execute('SELECT username FROM users WHERE username=?', (username))
  if checkUsername != 0:
    print('Username is not exist')
  else:
    print('Logged In!')

You have an error in your SQL syntax; check the manual that corresponds
to your MySQL server version for the right syntax to use near '?' at line 1