Why does an error occur when including an application in a Django project?

Question

Django

There is a project FirstProject. It has the FirstApp application.
Here is the list of INSTALLED_APPS from FirstProject/settings.py:

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'FirstApp',
]

Here is the error:
File "D:\DjangoVenv\FirstProject\FirstProject\urls.py", line 21, in
path('first/', include(FirstApp.urls)),
NameError: name 'FirstApp' is not
defined correct and what caused the error?
Thanks in advance!