Error
The REST API has been restricted to authenticated users.
hangjin jo you are blocking webhooks with some other plugin (security?) or code snippet.
Once you locate the plugin you need to white-list all webhooks/endpoints.
Example: if you use fCommunity plugin but you decided to make it Public (access not only for logged in users) then you need to enable fCommunity web hooks in order to use fCommunity etc.