“Proper authorization is required to access this resource”

The wp_remote_get() function is trying to access the website as an outsider. Meaning, it can’t bypass any htpasswd or security measure you might have had in place.

In order to solve this, simply remove any password protection you have on the page and your request should be accepted.

Note, in order to test if a page is protected, simply open it in a private browsing session to check.