Can you share your DATABASES configuration? Are you saying that you’re using SQLite as your db?
If so, I think that’s not going to work on Heroku. SQLite saves a file locally, but Heroku can move your application to a different dyno at any time. That means that any files that are created on a Heroku dyno aren’t guaranteed to be there in the future. That would wreak havoc on a SQLite database since it could “disappear” at any time.
If you’re running on Heroku, I believe you’ll need to be able to connect to a different database like Postgres.