When you set up the Google Play: Reply to Reviews Integration, the Client ID and Client Secret that you created in the OAuth client section of Google Play are not valid in AppFollow.
This means that the OAuth client has expired and needs to be refreshed. By default, the OAuth token is valid for 1 day. The Google Developer account owner may change the limit to 7 or 30 days. To complete the integration, the Google Developer account owner needs to refresh the OAuth client and provide a new Client ID and Secret.