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 you are following this guide https://www.home-assistant.io/integrations/alexa.smart_home/ when it gets to the test phase "click Configure test events, you can Create new test event using following data:"
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).
