title: Django "Dependency on unknown app" error even though the app folder exists
body: <p>I'm trying to run manage.py runserver or manage.py makemigrations, but I'm getting an error when I do. The error is: ValueError: Dependency on unknown app: backend.</p>
  
  <p>Here's the full error message:</p>
  
  <pre><code>Benjamins-MacBook-Pro:package-backend benhsu$ python3 manage.py makemigrations
  Traceback (most recent call last):
    File "manage.py", line 10, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
      utility.execute()
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/__init__.py", line 377, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/base.py", line 288, in run_from_argv
      self.execute(*args, **options.__dict__)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/base.py", line 338, in execute
      output = self.handle(*args, **options)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/commands/makemigrations.py", line 57, in handle
      loader = MigrationLoader(None, ignore_no_migrations=True)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/migrations/loader.py", line 48, in __init__
      self.build_graph()
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/migrations/loader.py", line 243, in build_graph
      parent = self.check_key(parent, key[0])
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/migrations/loader.py", line 168, in check_key
      raise ValueError("Dependency on unknown app: %s" % key[0])
  ValueError: Dependency on unknown app: backend
  </code></pre>
  
  <p>"backend" is the app I'm working on, and you can see in my folder that it exists. Here's my folder structure:</p>
  
  <p><img src="https://i.stack.imgur.com/XPVkG.png" alt="enter image description here"></p>
  
  <p>Here's the relevant part of my settings.py file:</p>
  
  <pre><code># Application definition
  
  AUTH_USER_MODEL = 'backend.User'
  
  INSTALLED_APPS = (
      'django.contrib.admin',
      'django.contrib.auth',
      'django.contrib.contenttypes',
      'django.contrib.sessions',
      'django.contrib.messages',
      'django.contrib.staticfiles',
      'rest_framework',
      'backend'
  )
  </code></pre>
  
  <p>I don't understand why it's not working since the backend app exists in my folder.</p>
  
  <p>I'd appreciate any help!</p>