This is a simple python template that uses Flask to build a webhook for Facebook's Messenger Bot API.
Read more in my tutorial that uses this repository.
The #1 error that gets reported in issues is that facebook returns an error message (like above) when trying to add the heroku endpoint to your facebook chat application.
Our flask application intentionally returns a 403 Forbidden error if the token that facebook sends doesn't match the token you set using the heroku configuration variables.
If you're getting this error, it likely means that you didn't set your heroku config values properly. Run heroku config from the command line within your application and verify that there's a key called VERIFY_TOKEN that has been set, and that it's set to the same value as what you've typed into the window on facebook.