def _validate_username(self, username, verbose_field_name, database):
        """
        Validate username. If invalid, return a string error message.
        """
        if self.username_is_unique:
            try:
                self.UserModel._default_manager.db_manager(database).get_by_natural_key(
                    username
                )
            except self.UserModel.DoesNotExist:
                pass
            else:
                return f"Error: The {verbose_field_name} is already taken."
        if not username:
            return f"{capfirst(verbose_field_name)} cannot be blank."
        try:
            self.username_field.clean(username, None)
        except exceptions.ValidationError as e:
            return "; ".join(e.messages)

        Validate username. If invalid, return a string error message.
        
babybuddy/management/commands/createuser.py