From the error message it sounds like your API key is not valid. If you have updated your API key and it still is giving you the error then you should file a support ticket with mendix support.
Please note that there are two distinct ATS API keys. One is the API key you can use from CICD tools to invoke ATS tests – not what you need here and there is the Mendix API key which is used to retrieve the stories.
None of them have anything to do with the projects/apps, they are retrieved via another channel.
I hope this helps!