class User(AbstractUser):

    REQUIRED_FIELDS = []
    USERNAME_FIELD = 'email'
    email = models.EmailField(
        _('email address'),
        max_length=150,
        unique=True,
        help_text=_('Required. 150 characters or fewer. Must be a valid email address.'),
        error_messages={
            'unique':_("A user with that email address already exists."),
        },
    )

# custom user model
AUTH_USER_MODEL = 'workoutcal.User'

(workout) n155-p250:workout sahandzarrinkoub$ ./manage.py createsuperuser
Email address: sahandz@kth.se
Password:
Password (again):
Traceback (most recent call last):
  File "./manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/core/management/__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/core/management/__init__.py", line 365, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/contrib/auth/management/commands/createsuperuser.py", line 59, in execute
    return super().execute(*args, **options)
  File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/core/management/base.py", line 335, in execute
    output = self.handle(*args, **options)
  File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/contrib/auth/management/commands/createsuperuser.py", line 179, in handle
    self.UserModel._default_manager.db_manager(database).create_superuser(**user_data)
TypeError: create_superuser() missing 1 required positional argument: 'username'

class CustomUserManager(BaseUserManager):
    def _create_user(self, email, password, is_staff, is_superuser, **extra_fields):
        now = timezone.now()
        if not email:
            raise ValueError('email must be set')
        email = self.normalize_email(email)
        user = User(email = email, is_staff=is_staff,
                    is_superuser=is_superuser, date_joined=now,
                    **extra_fields)
        user.set_password(password)
        user.save()
        return user

    def create_user(self, email, password, **extra_fields):
        return self._create_user(email, password, False, False, **extra_fields)

    def create_superuser(self, email, password, **extra_fields):
        return self._create_user(email, password, True, True, **extra_fields)

class User(AbstractUser): # in your custom user
    #...
    objects = CustomUserManager()
    #...