client_credentials is not an approriate token for this endpoint*
You need a user token representing the user you wish to read the subscriptions of.
So you need to ask jankos to go to your website and login with Twitch and store the token
Additionally jankos is not the broadcasters ID but their login (use the ID returned from the Get Users Endpoint).
You have generated and used the wrong kind of oAuth token.
*Except when the ClietnID is an extension and the extension is installed/active on the target channel