Your code doesn’t work because you’re adding &callback=? to the end of it for some reason, which isn’t a valid param and the question mark at the end is malforming the querystring.
Additionally, you’re not sending the v5 header so the endpoint defaults to v3 which doesn’t exist any more and has long since been deprecated (just as v5 will be in the near future as it’s already marked as deprecated, which is why is why you’ve been encouraged to use Helix).
If you logged the body of the response you would have seen error messages indicating all of this