Sounds like you are not redirecting the user.
You literally need to send the users web browser to Twitch.
The server shouldn’t be fetching itself.
This is suggested by the error saying XMLHttpRequest
Which suggests you are using JS Fetch or XMLHttpRequest and not issuing a HTTP Redirect to redirect the browser.
You either need a <a href="the link to twitch"> or to redirect the user to the link to twitch