Youtube analytics API 403 error brand account

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 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:

error 403 org_internal

and of course the request details are all fulfilled; there shouldn't be a problem with authorization (I'm not using a service account), scopes, etc...

I can use at least my main account for the API authorization... I don't know why I should be coming up on these problems.

Edit: please help, I've spent another full day working on this, nothing works! Maybe there is some testing that can be done in the OAuth explorer?