I ran into the same issue and spent many a wasted hour searching for an answer.
To keep anyone else from doing the same, here is what i did. (short answer: keep going)
If it comes back with the above mentioned response of “type”: “INVALID_AUTHORIZATION_CREDENTIAL”,
As long as you have followed the guide up to this point and generated the LONG_LIVED_ACCESS_TOKEN continue on to the next step in the guide it will work.
I have attached the two size home assistant images below, to integrate into the Alexa interface (if they show up).