Django: (fields.E300) Field defines a relation with 'Class' model , which is either not installed, or is abstract

Question

Class model is a foreign key in File model I have no idea what causes the error above,I searched it and it mostly occurs when working with models in different apps, but that is not case in this situation, since both models are in the same models.py file.

models.py

from asyncio.windows_events import NULL
from http.cookiejar import FileCookieJar
from django.db import  models
from django.utils import timezone
from django.contrib.auth.models import AbstractBaseUser,PermissionsMixin, BaseUserManager
class Class (models.Model):
    name=models.CharField(max_length=256);
    def __str__(self):
        return self.name;

class File (models.Model):
    file=models.FileField();
    name=models.CharField(max_length=255);  
    class_name=models.ForeignKey(Class,on_delete=models.CASCADE)
    def __str__(self):
        return self.name;

class Class (models.Model):
    name=models.CharField(max_length=256);
    def __str__(self):
        return self.name;

class CustomUserManager(BaseUserManager,):
        def create_superuser(self,email,name,surname,password,**other_fields):
                other_fields.setdefault('is_staff',True)
                other_fields.setdefault('is_superuser',True)
                other_fields.setdefault('is_active',True)
                if other_fields.get('is_staff') is not True:
                    raise ValueError('super  user must be assigned to is_staff= True')
                if other_fields.get('is_superuser') is not True:
                    raise ValueError('Super user must be assigned to is_super_user=True')
                return self.create_user(email,name,surname, password,**other_fields)   


        def create_user(self,email,name,surname,password,**other_fields):
         if not email:
          raise ValueError(('You must provide an email address'))
         email=self.normalize_email(email); #for example ignore the case sensitivity
         user=self.model(email=email,name=name,surname=surname,**other_fields)
         user.set_password(password)
         user.save()
         return user;

     
class NewUser(AbstractBaseUser,PermissionsMixin):
        email=models.EmailField(max_length=255,unique=True,default=NULL,)
        name=models.CharField(max_length=255)
        surname=models.CharField(max_length=255)
        is_staff=models.BooleanField(default=False)
        is_active=models.BooleanField(default=True) 
        is_teacher=models.BooleanField(default=False)
        classes=models.ManyToManyField(Class)
        objects=CustomUserManager();
        USERNAME_FIELD='email'
        REQUIRED_FIELDS=['name','surname','is_teacher']

        def __str__(self) :
            return self.name





class Request(models.Model):
    requested_class=models.ForeignKey(Class,on_delete=models.CASCADE)
    requesting_student=models.ForeignKey(NewUser,on_delete=models.CASCADE)  

    def __str__(self):
        return self.requesting_student.name


# Create your models here.