I understand; as it turns out, there's an existing issue with the Tickets API with regard to OAuth authentication. The teams is currently working on resolving this, but for right now the Tickets API will only work with hapikey authentication. I'll keep this topic updated when I have more information on the resolution of this issue. Thanks again for your patience here!
Can you successfully create/delete/read tickets via the API when using your portal's hapikey? If so, it's likely due to the scopes your OAuth refresh/access tokens have. Can you try going through the OAuth flow again?
The Tickets API is currently in developer preview, and should be accessible in all portals. If you're having issues accessing certain Tickets API methods, post a link to an example record in your test portal so that I can take a closer look.
@Derek_Gervais We are experiencing similar issue: we can't create, delete (Must have scope TICKETS_WRITE) nor load tickets (Must have scope TICKETS_READ) over API when authenticated with OAuth.