I’m having this exact same issue as another community member:
"Quick summary: What specific settings or permissions are required in this case so that my second YouTube channel (and so forth) is properly recognized to be within my organization and accessible via my internal only facing google application?
I have a single google workspace account with a single user. As the admin user I created two YouTube channels. The first YouTube channel was created as a non-Brand Channel and tied directly to the same workspace account used to create the channel. The second channel was automatically created as a Brand Channel because YouTube only allows the first channel to be non-brand while requiring all subsequent channels on the same google workspace account to be Brand Channels. This is true for both workspace and non-workspace google accounts.
For the purpose of managing both YouTube channels through the YouTube API I created a new google project on console.cloud.google.com. Within that project created an application under the OAuth consent screen named App1. I made this application internal so to avoid having to go through the google verification process for this application. When I attempt to obtain a refresh token via a script using the API the authentication screen asks me to choose between my regular google workspace account and the name of the second YouTube channel. If I select the workspace account everything works fine and I receive the token. If however I select the second YouTube channel I receive an “Accessed Blocked” screen with “Error 403:org_internal” error at the bottom.
The problem is that the second YouTube channel is being seen as external to my organization due to it being a brand YouTube channel despite the fact that it is within my single google workspace account & organization. What specific settings or permissions are required in this case so that my second YouTube channel is properly recognized to be within my organization and accessible via my internal only facing google application?"