Why am I getting a boot timeout for flask app in Heroku?

Question

I'm having trouble deploying my flask app to heroku. It keeps giving me a Error R10 :

Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch

I don't understand why though because my Procfile defines a port :

web: python3 run.py runserver 0.0.0.0:5000

Even when my Procfile is just :

web: python3 run.py 

It still doesn't work and gives me the same error. If needed here is my run.py file :

from serv import app

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

Thanks in advance!!!