Web Process Failed To Bind To $port Within 60 Seconds Of Launch

Can someone explain to me this error message on Heroku? App works fine locally but has never succeeded online heroku[slug-compiler]: Slug compilation started heroku[slug-compiler]

Solution 1:

You need to specify host='0.0.0.0' in app.run(). Unless told otherwise, Flask binds to "localhost", which means it won't bind to an externally visible interface.

See Flask - configure dev server to be visible across the network