[Answer]-CommandError: notifications.notification: 'recipient defines a relation with the model 'auth.User'

in django-notifications 0.6.0 https://github.com/brantyoung/django-notifications/blob/0.6/notifications/models.py:

recipient = models.ForeignKey(User, blank=False, related_name='notifications')

line 87. Recipient refers auth.User, not your custom user

You need version 0.6.2 or 0.6.1