Created a brand new Django Project, attempting to run the server generates an Improperly Configured Error
Going off of the comments here.
If "env | grep DJANGO_SETTINGS_MODULE" returns empty, it means you have to set an environment variable stating where your settings.py file is located.
This can be done by doing the following:
export DJANGO_SETTINGS_MODULE=mysite.settings
Be sure to replace "mysite" with the name of your app!
I have been working on a Django Project for a bit, until I took a break. One month later I dug back in to the project and went to run the server. I received an error:
 django.core.exceptions.ImproperlyConfigured: Requested setting DEBUG, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.
I figured I must have tweaked something by accident. So I created a brand new Django Project (using a virtual environment), and just went to test the server. I received the same error. I tried the python manage.py shell solution listed in another answers but to no avail. 
If it helps I'm on Linux with Django version 2.1.5 and Python 3.6.
Edit:
If anyone encounters something similar I found using python3 manage.py runserver works in place of using django-admin. Per Greg's answer below, I did begin to receive a new error ModuleNotFoundError: No Module named "mysite" exists. I will continue to search for an answer on that front.