app_name error happens from django 2.0 or over.
Before 2.0 ver, in the urlpattens of project urls.py, you just added
namespace in the url().
# project_name/urls.py urlpatterns = [ .... url(r'^sample/', include('sample.urls', namespace='sample')) ]
And you don’t need to add
app_name in other file.
but from django ver 2.0, you must add
app_name in application urls.py.
For instance, according to example above, you must add following line in sample/urls.py (sample is application name).
# sample/urls.py app_name = 'sample'