heroku h10 error, how to fix?

Question

Heroku

2020-05-14T23:35:18.978098+00:00 heroku[run.9440]: State changed from starting to up
2020-05-14T23:35:22.901368+00:00 heroku[run.9440]: State changed from up to complete
2020-05-14T23:36:07.120118+00:00 heroku[web.1]: State changed from crashed to starting
2020-05-14T23:36:12.339207+00:00 heroku[web.1]: State changed from starting to crashed
2020-05-14T23:36:12.279143+00:00 app[web.1]: bash: gunicorn: command not found
2020-05-14T23:36:14.060671+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=sheltered-cliffs-70556.herokuapp.com request_id=80629ad5-281f-4ba7-badf-479beb9722f5 fwd="89.223.47.198" dyno= connect= service= status=503 bytes= protocol=https
2020-05-14T23:36:15.504119+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/favicon.ico" host=sheltered-cliffs-70556.herokuapp.com request_id=167de43b-5c6d-4f04-bce1-8a27e7dc32d0 fwd="89.223.47.198" dyno= connect= service= status=503 bytes= protocol=https

Gita project