Using the Django admin you need to create a SocialApp listing your Facebook app credentials. Make sure that this app is attached to the proper site (as in, django.contrib.sites.models.Site).
In your case, there needs to be a django.contrib.sites.models.Site instance with id=2 (check the sites admin) that is listed as a site for the SocialApp.
If either the SocialApp is missing, or if it is created but not attached to a site matching your settings.SITE_ID, then allauth does not know what app to pick, resulting in the error message you listed above.