RuntimeError: Model class address.models.Country doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS. code example

Example: Model class django.contrib.sites.models.Site doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS

Just add 'django.contrib.sites', to INSTALLED_APPS and set SITE_ID = 1 in your settings.py file.