AUTH_USER_MODEL refers to model '%s' that has not been installed" % settings.AUTH_USER_MODEL django.core.exceptions.ImproperlyConfigured: AUTH_USER_MODEL refers to model 'app.User' that has not been installed

from django import forms
from django.contrib.auth import get_user_model
from django.contrib.auth.forms import UserCreationForm
from django.conf import settings


class SignUpForm(UserCreationForm):
    first_name = forms.CharField(max_length=30)
    last_name = forms.CharField(max_length=30)
    email = forms.EmailField(max_length=254)

    class Meta:
        model = get_user_model()
        fields = ('username', 'password1', 'password2', 'email')

from django.db import models
from django.utils import timezone
from django.db.models.signals import post_save
from django.dispatch import receiver
from django.contrib.auth.models import AbstractBaseUser, BaseUserManager, PermissionsMixin
from django.conf import settings

class UserManager(BaseUserManager):
    def create_user(self, email, username, password, first_name, last_name):
        if not email:
            raise ValueError("enter an email")
        if not username:
            raise ValueError("enter a username")
        if not password:
            raise ValueError("enter a password")

        user = self.model(
             email = self.normalize_email(email),
             username = username, first_name = first_name, last_name = last_name)
        if user.occupation == "admin": user.is_staff = True
        user.set_password(password)
        user.save()
        return user

    def create_superuser(self, email, username, password, first_name, last_name):
        user = self.create_user(email, username, password, first_name, last_name)
        user.is_superuser = True
        user.is_staff = True
        user.save()
        return user

class User(AbstractBaseUser, PermissionsMixin):
    email = models.EmailField(unique=False)
    username = models.CharField(max_length=25, unique=True)

    is_staff = models.BooleanField(default=False)
    is_active = models.BooleanField(default=True)

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