Trouble to deploy flask application on heroku

I'm having trouble to deploy my flask application on heroku, my directory tree is looking like down below:

├── app
│   ├── errors.py
│   ├── forms.py
│   ├── __init__.py
│   ├── __init__.pyc
│   ├── models.py
│   ├── __pycache__
│   ├── routes.py
│   ├── static
│   ├── storage.db
│   └── templates
├── migrations
├── Procfile
├── README.md
├── requirements.txt
├── run.py
└── runtime.txt

  $ cat runtime
  python-3.8.5

  $ cat Procfile 
  web: gunicorn: app:app

I'm quite confuse on the configuration of Procfile, should I call run.py (The archive I use to call locally with "flask run") or app (The folder where init.py calls flask instance and sql_alchemy instance)?

Anyway, both configurations returned the error below:

$ heroku logs --tail

heroku[web.1]: State changed from crashed to starting
heroku[web.1]: Starting process with command `gunicorn: app:app`
app[web.1]: bash: gunicorn:: command not found
heroku[web.1]: Process exited with status 127
heroku[web.1]: State changed from starting to crashed

gunicorn is already configured at requirements.py