python 3.x – TypeError: __str__ returned non-string (type User)

I’ve Been Trying to Work This Model But In Class Paid Videos I’m getting Error

makemigrations is working , migrations working but when I runserver and opensenter code here the site IT shows me error I tried doing which I marked in code

from django.core.exceptions import ValidationError
from django.core.validators import MaxValueValidator

# Create your models here.

def Validate(A):
    if post.objects.filter(post.post_type !='Paid'):
        raise ValidationError('If You Want to Set Amount On Your Content You Have to Set Paid Content Type')

class paid_videos(models.Model):
    paid_id = models.AutoField(primary_key=True)
    post_timestamp = models.ForeignKey(post, on_delete=models.CASCADE)
    post_amount = models.PositiveIntegerField(validators = (MaxValueValidator(999),Validate))

    # def __str__(self): <- <- I tried this
    #     return self.post_timestamp #also for other 2 fields