It’s yours, it’s just an Opaque ID, not an actual User ID.
Are you using a proper OAuth token? If you’re attempting to use anything from your extensions front end then that’s not an OAuth token and wont work.
If you want to get your Twitch ID you can use the endpoint specified here in the docs: Reference | Twitch Developers
You don’t have to use an OAuth token, you can simply use your client id, and specify your username as the login querystring param.