So, I upgraded my django version from 1.11 to 2.2 and ran python manage.py runserver. I get the error:
Model class <project_app_name>.models.<model_name> doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.
Obviously, all apps are registered including the <project_app_name>. So, the answers that try the solution of adding the app to the installed apps don't apply here.
Any help would be appreciated