Hello,
Thanks for reaching out. The behavior you're describing sounds like what would happen in the event that you attempted to access the UI using a Google account that does not have access to your Authorized Buyers account. When an account is initially configured, a few Google accounts are granted access initially, which may then grant access to others–it sounds like the account you're logged into was either never granted access or had that access revoked. In other words, you are likely unable to access the UI for that account for the same reason that the service account you're attempting to add can't access the API for that account.
These support forums are probably not the channel best suited to helping you to resolve this issue because we don't manage user accounts here, and sharing your account details in a public setting is ill-advised. Assuming you have one, I'd recommend getting in touch with your Technical Account Manager to get this resolved. Otherwise, if you don't have one, the same team can be contacted via this
support form.
Regards,
Mark