Why did my inspection fail to complete?
The usual reasons are that the page blocks automated requests, sits behind a firewall or bot filter, returns something other than a 200, presents a certificate that will not verify, or is larger than the download limit. Each of those produces its own message rather than a generic error, because the reason is usually itself a finding.
Using The Inspection
Answer it for your own page
The inspection is free, needs no account, and takes a few seconds. It reports what is actually wrong with a page rather than what is generally true about pages.