I'm stuck as well. Unfortunately it seems like Google is no longer issuing API keys unless you have a domain. Could anyone more knowledgeable confirm we are doing this correctly? I've linked a stackoverflow forum about the Google API that seems to say the error is intended behavior, and provided screenshots of my setup in google.
This specific error doesn't seem to be related to domain ownership.
I have a domain I can use for testing, I got it verified, and added to the "Authorised domains" section of the OAauth consent screen.
I tried adding credentials using the method posted on github.
Personal API Keys · jdf76/plugin.video.youtube Wiki · GitHub
When I tried to add credentials I got the exact same pop-up error, even with a verified domain added.
I haven't had a chance to test yet, but I suspect it's to do with selecting "Other UI (e.g. Windows, CLI tool)" from the dropdown menu.
This option no longer exists when you try to add credentials via the Credentials side menu entry. Instead, for application type, you get options like "TV's and Limited input devices" and "Desktop app".