I've tried running the analytics/reporting APIs with my main account (using my local machine's terminal to run python scripts from the YT API Github), and came up with only blank reports (a header and no data or only zeros with the analytics API) multiple times. With my brand accounts, I end up with an error 403 org_internal when trying to authorize via OAuth authentication flow.
I've also tried following the steps laid out here:
YouTube APIs - Access mutiple youtube channels (Brand Accounts) using Google Admin account
Once I get to step 6, I get the following error:
and of course the request details are all fulfilled; there shouldn't be a problem with authorization (I'm not using a service account), scope, etc... maybe there's something I haven't thought of, but since I can use at least my main account for the API authorization, where there is data, I don't know why I should be coming up on these problems.