Hi All
It seems I confused the access type of the MCC account with the test account
So, to recap for anybody possibly reading this you need:
1. Production MCC account which will provide develop-token. Initially, this developer token can be used against test-accounts.
2. Then you can create A manager Test account
3. Then you can add a test client under that manager test account.
4. Then you can make requests with the developer token on those client ids.
I also had another error regarding DEVELOPER TOKEN access not granted for project X.
Once you make requests under a certain project (Google Cloud I think) you cannot make using a different one.