Django - null value in column violates not-null constraint in Django Admin

Just add null=True in all your Fields check https://docs.djangoproject.com/en/3.1/topics/migrations/#postgresql

Looking at your models you shouldn't have field category_id in any of your tables. Perhaps you changed your models but did not alter tables in the database. Now when you create an entry, Django does not fill fields it doesn't know about of and this creates an error. You should remove unneeded fields from your tables. Or if it is possible you can drop the whole database and run manage.py syncdb from scratch.