I have just create a setup here on my side,
I misspelled on purpose the page-access-token on credentials, and I noticed that:
Even if the page-access-token is wrong, the callback url will be added successfully on facebook, (because the Verify Token would still give the right answer )
After that I ran some tests on FB,
I started the server on debug mode: I would get 200 OK on my ngrok tunnel, my model would predict the answer but wouldn’t fire it, so I advise you to check if the page-access-token is correct or to generate a new one
Also, run it on debug mode << rasa run --credentials credentials.yml --debug >> so you can have more information.
cheer