how to change USERNAME_FIELD in Django 3.0.8. without creating a custom user?

Question

i know same Question has already been asked and tried that solution by putting User.USERNAME_FIELD = 'email' in __init__.py file but it gives error

raise AppRegistryNotReady("Apps aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.

settings.py

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