I already change the name correctly to Procfile with capital P but the problem still occurs.
this is what Heroku says in the terminal:
here is my project:
and here is the code in Procfile:
web: gunicorn app_with_handler.py:app --log-file -
did I miss something or are there anything I should try next?
Update: solve the problem by deleting the Procfile, commit, and push to heroku. Then create the same exact Procfile again, commit and push to heroku again. This way seems to work for me.