AUTH_USER_MODEL refers to model 'Socialmedia.User' that has not been installed

Question

I created my custom user model and I have a function which allows users to sign in . I think my custom user model is incompatible with my function because it wouldn't allow me to do User.objects.get(username=username) . How can I fix this so I can use the db?

AUTH_USER_MODEL refers to model 'Socialmedia.User' that has not been installed

my views.py:

def sign_up(request):
if request.method == 'POST':

    username = request.POST.get("username").lower()
    password = request.POST.get("password")
    email = request.POST.get("email")
    password2 = request.POST.get("password2")

    users = User.objects.get(username=username)

    if users is not None:
        messages.error("The user already exists, please select a different username")
    else:
        user = User.objects.create_user(username=username, password=password, email=email)
        user.save()

return render(request, 'usermgm/costume_register.html')

models.py:

in models i created
imports:

from django.contrib.auth.models import User
from django.contrib.auth.models import AbstractBaseUser, BaseUserManager

models.py:

class UserManager(BaseUserManager):
def create_user(self, email, password=None, **extra_fields):
    if not email:
        raise ValueError('The Email field must be set')

    email = self.normalize_email(email)
    user = self.model(email=email, **extra_fields)
    user.set_password(password)
    user.save(using=self._db)
    return user
def create_superuser(self, email, password=None, **extra_fields):
    extra_fields.setdefault('is_staff', True)
    extra_fields.setdefault('is_superuser', True)

    if extra_fields.get('is_staff') is not True:
        raise ValueError('Superuser must have is_staff=True')

    if extra_fields.get('is_superuser') is not True:
        raise ValueError('Superuser must have is_superuser=True')

    return self.create_user(email, password, **extra_fields)




class User(AbstractBaseUser):
email = models.EmailField(max_length=255, unique=True)
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=30)
is_active = models.BooleanField(default=True)
is_staff = models.BooleanField(default=False)
is_superuser = models.BooleanField(default=False)

USERNAME_FIELD = 'email'
REQUIRED_FIELDS = ['first_name', 'last_name']
objects = UserManager()
def __str__(self):
    return f'{self.first_name} {self.last_name}'

settings.py:

AUTH_USER_MODEL = 'Socialmedia.User'

I've tried: adding the User model to init.py file with all.