As you stated that you have sent the request without authentication information, the URL will return 401 status code and hence the error message is displayed.
If you want to treat this status as UP, kindly configure 401 as an Accepted HTTP status code as shown in the image. Otherwise, you'll have to give the necessary authentication information to make this work.
Please navigate to Web > for the selected monitor click on the hamburger icon (far right corner) > choose edit > enter 401 in the field "Accepted HTTP status codes".
I hope this answers your query. If you're still facing an issue, please reply back to this same thread and we'll look into it.
Product Developer, Site24x7