Django - ModuleNotFoundError: No module named 'backend'

Question

I have problem with "ModuleNotFoundError" in my Django app.

This problem start when i write serializers.py and other files in my app.

All code you see it in my Github

This code is without syntax errors because I use PyCharm IDE.

When I start server I get this error:

PyCharm IDE with terminal after run python server

I would appreciate any comments on where I made a mistake.

I expect my code to work correctly.