Heroku error code=H10 status=503 when deploying python flask app

Question

I get this error code:

rashed 2020-08-27T00:35:29.293812+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=song-recommender-michaelwzhu.herokuapp.com request_id=516f57c9-167b-47c0-86fb-3258aa046768 fwd="24.4.72.198" dyno= connect= service= status=503 bytes= protocol=https

My python flask app works when hosted locally, but displays application error when deployed on Heroku.

My app.py file:

app = Flask(__name__)

*code*

if __name__ == '__main__':
app.run(debug=True)

My requirements.txt file:

click==7.1.2
Flask==1.1.2
gunicorn==20.0.4
itsdangerous==1.1.0
Jinja2==2.11.2
MarkupSafe==1.1.1
Werkzeug==1.0.1

My Procfile:

web: gunicorn app:app