How to fix "ImproperlyConfigured" erros in django rest_framework urls.py - python-3.x

Question

I am trying to figure out how django rest framework works and write some code. But i am getting
django.core.exceptions.ImproperlyConfigured: The included URLconf 'tutorial.urls' does not appear to have any patterns in it. If you see valid patterns in the file then the issue is probably caused by a circular import
what is circular import? I can't understand. I created a app called quickstart inside i create serializers.py file.
quickstart/serializers.py
from django.contrib.auth.models import User, Group
from rest_framework import serializers
class UserSerializer(serializers.HyperlinkedModelSerializer):
class Meta:
model = User
fields = ('url', 'username', 'email', 'groups')
class GroupSerializer(serializers.HyperlinkedModelSerializer):
class Meta:
model = Group
fields = ('url', 'name')
quickstart/views.py
from django.contrib.auth.models import User, Group
from rest_framework import viewsets
from quickstart.serializers import UserSerializer, GroupSerializer
class UserViewSet(viewsets.ModelViewSet):
"""
API endpoint that allows users to be viewed or edited.
"""
queryset = User.objects.all().order_by('-date_joined')
serializer_class = UserSerializer
class GroupViewSet(viewsets.ModelViewSet):
"""
API endpoint that allows groups to be viewed or edited.
"""
queryset = Group.objects.all()
serializer_class = GroupSerializer
quickstar/urls.py
from rest_framework import routers
from .views import UserViewSet, GroupViewSet
router = routers.DefaultRouter()
router.register(r'users', views.UserViewSet)
router.register(r'groups', views.GroupViewSet)
urlpatterns = router.urls
testing/urls.py
from django.contrib import admin
from django.urls import path, include
urlpatterns = [
path('', include('quickstart.urls')),
# path('api-auth/', include('rest_framework.urls', namespace='rest_framework')),
path('admin/', admin.site.urls),
]
can you please describe why this error happened frequently?
Thank you!