You have multiple authentication backends configured and therefore must provide the `backend` argument.

Raise code

rm(self, perm, is_active=True, include_superusers=True, backend=None, obj=None):
        if backend is None:
            backends = auth._get_backends(return_tuples=True)
            if len(backends) == 1:
                backend, _ = backends[0]
            else:
                raise ValueError(
                    'You have multiple authentication backends configured and '
                    'therefore must provide the `backend` argument.'
                )
        elif not isinstance(backend, str):
            raise TypeError(
                'backend must be a dotted import path string (got %r).'
                % backend
            )