If the page returns a HTTP error, we can’t analyze it. A HTTP error code is a 3-figure code corresponding to the state of the page.
We can’t analyze the page if it returns a code that doesn’t start with a 2 (2xx).
Get all HTTP error codes list on the MDN documentation.
NB: It’s possible that on some servers a 403 code is only returned by our robot. This is probably due to a security system installed on your website which you’ll need to configure to authorize our analysis robot.