Trying to get channel's subscriptions only gives error

API
4

For regular user oAuth

  1. Generate a URL
  2. Redirect the user to that URL
  3. They accept (or decline) allowing access to their account to your Client ID
  4. They return to your website with a code (or error message)
  5. Exchange the code for an access token (using the post request you posted)

To get subscriber data you need permission from the broadcaster, which will require this “oAuth loop” to get delegated access.