Flask app dont start on heroku server

1.Procfile should be without any extension.

2.Content of Procfile should be: web: gunicorn app:app --preload

Note:usually --preload is used when the size is more.

Is there actually a running dyno called web? It looks like you might have scaled your web dynos down to 0:

Use a ps:scale command like this to scale your web dynos to at least 1:

heroku ps:scale web=1

You can use

heroku ps

to confirm that your web dyno is running.