I landed on this post via Google search. My problem was running tests that blew up with the error:

RuntimeError: Model class app.taxonomy.models.Term doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.

This was running on Python 2.7.x with absolute imports. As mentioned by Colton Hicks in the comments, below, this can also happen with Python 3 (pytest 3.2.3 with Django 1.11.4).

In my tests.py:

from __future__ import absolute_import
[...]
from .models import Demographics, Term

After changing the relative import to an absolute import the problem went away:

from taxonomy.models import Demographics, Term

HTH

I have django debug toolbar installed and this was actually causing the/my problem.
INSTALLED_APPS (in settings.py) needs the entry ‘django.contrib.sessions’. Make sure to run migrate after adding.

Try adding the app_label = 'yourApp' in the models Meta class:

class Meta:

    app_label = 'yourApp'

I got the error above. However my problem was the in the urls.py. I was following PyDanny cookiecutter django recipe. My error was to put in the urls.py this line:

    url(r'^demo/', include('project.demoapp.urls', namespace='demoapp')),

when I corrected to this:

    url(r'^demo/', include('demoapp.urls', namespace='demoapp')),

all was well. I also changed my local apps (I did this first and so the critical error was the url misconfiguration):

LOCAL_APPS = [
    # Your stuff: custom apps go here
    'demoapp.apps.DemoAppConfig',
]

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

Upgraded Answer for Django>=4.0 // 2022

Add Django’s Sites framework and FlatPages Framework to your INSTALLED_APPS and set SITE_ID in your settings.

INSTALLED_APPS = [
    ...
    'django.contrib.sites',
    'django.contrib.flatpages',

]

SITE_ID = 1

Your tests should work like a charm

Django 4.1+ (2023)

After almost an hour digging, what solved for me was this:

INSTALLED_APPS = [
    ...
    'django.contrib.sessions',
]

No need for SITE_ID or additional INSTALLED_APPS entries.

This error message typically occurs when Django encounters a model class that doesn’t have an explicit app_label defined and is not included in the INSTALLED_APPS list in your Django project’s settings.

To resolve this issue, you can follow these steps:

1. Open your Django project’s settings file (settings.py).

2. Locate the INSTALLED_APPS list, which should contain the names of all the installed apps in your project.

3. Make sure that the 'django.contrib.sessions' app is included in the INSTALLED_APPS list.

   INSTALLED_APPS = [
       # ...
       'django.contrib.sessions',
       # ...
   ]
   

If you find that ‘django.contrib.sessions’ is missing, add it to the list as shown above and save the file.
If the app is already included in the INSTALLED_APPS list and you still encounter the error, there might be another issue with your project’s configuration. In that case, it’s worth verifying that you have the correct versions of Django and its dependencies installed, as compatibility issues can sometimes cause such errors.

Additionally, if you have custom model classes that don’t have an explicit app_label defined, you should add the app_label attribute to those model classes. For example:

class Session(models.Model):
    # model fields
    class Meta:
        app_label = 'sessions'

By specifying the app_label attribute, you explicitly declare the app label for the model class, which helps Django locate and recognize it correctly. Replace ‘sessions’ with the appropriate label for your app.

Remember to restart your Django development server after making any changes to the settings file to ensure that the modifications take effect.

This error occurred because I had created a new app folder for a subset of sites related to another feature. This needed to be added to my INSTALLED_APPS in settings.py

Everything worked as expected after I made a migration

python manage.py migrate

Good luck