The Site object for your Django project is missing. Each Django project has a Site object which contains the site's name and domain. It is usually automatically created when creating a Django project but in your case it seems that didn't happen.
To fix it:
Open the Django shell for your site (python manage.py shell).
Type the following:
>>> from django.contrib.sites.models import Site
>>> Site.objects.create(name='example.com', domain='example.com')
If you want to change these values later, go to your admin panel (/admin/) and edit the site object in the section Sites.
Hope this is helpful!!