You don’t really need the sites
framework if you only run one site from the project, so the easiest fix would be to remove the following item from your INSTALLED_APPS
and the error should go away:
'django.contrib.sites'
You can also re-create the missing Site object from shell. Run python manage.py shell and then:
from django.contrib.sites.models import Site
Site.objects.create(pk=1, domain='www.example.com', name="example.com")