Hi Ben,
Thank you for reaching out. The error USER_PERMISSION_DENIED is observed when you are not authorized to access customer account or the link between the manager and client account was removed. If you are using Google Ads API, could you please confirm if you have set the login-customer-id in your requests header to match the manager you're logging in as? You may refer to this blog post for more information about the new required header for Google Ads API requests. Also, please make sure if your OAuth credentials are created for a user having access to the client customer ID specified or any of its manager accounts. If the problem still persists, could you please share the user's email address authenticating the call and the complete request and response logs along with client customer id for me to investigate further? Please use Reply privately to author option while sharing the details.
Regards,
Nikisha Patel, Google Ads API Team