An incorrect Issuer in the payload of the client_secret can cause an "invalid_client" response. So check your Issuer value, as well. It should be the 10-character Team ID associated with your developer account.
surface0086
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution