Within Lighthouse, this error comes from gatherRunner.assertPageLoaded. This is called right after we finish loading the page for a pass (there are 3 passes by default). It looks for the HTML resources's network record (via some URL comparison).

If it fails to find a match, we error. Or if we find a match, but the network record is marked as failed, then we error.

There are a few cases in which this error shows up.

1. Server failure with initial request URL: The HTML request was matched but determined to be failed. This happens with incorrect server configurations, when the server does not reply at all, or replies with an empty/otherwise invalid response. Examples: Unable to load the page: timeout reached - Server misconfigured #2459 (redirect configuration was botched, resulting in a error page if you navigate to the HTTP regularly in chrome) DevTools Error: Unable to load the page: net::ERR_CONNECTION_RESET #2500 (TLS handshake error) DevTools Error: Unable to load the page: net::ERR_EMPTY_RESPONSE #2646 (a local server replies with an empty response)
   Solution: Fix your server to return valid HTML documents with successful status codes.
2. Server doesn't offer HTTP: The host doesn't offer a site at the HTTP URL at all. Example: Site Redirects HTTP Traffic to HTTPS shouldn't stop the execution if failling #1978
   Solution: Change your server to redirect HTTP traffic to HTTPS
3. HSTS: The host is using HSTS, and Chrome can't allow the browser to attempt to navigate to HTTP. Example: Unable to load the page: timeout reached - HSTS #2465
4. about:blank: Audits was started on about:blank somehow. Canonical issue: DevTools Error: Unable to load the page: timeout reached (about:blank) #2362. This should be fixed on the DevTools side, and will ship in m61, but can still happen occasionally for unknown reasons. If this happens consistently on a particular URL for you, please file a separate issue specifying the URL.
   Solution: Restart Chrome and run Lighthouse again. If problem persists with a particular URL, file a new issue here with the title "Reproducible about:blank errors"
5. No document request found: Lighthouse was unable to identify any network request as the main HTML resource. Example: Extension Error: Unable to load page: no document request found #3496
   Solution: Restart Chrome and run Lighthouse again. If problem persists with a particular URL, file a separate issue with the title "Cannot identify main document"
6. Bad TLS/certificate interstitial. Lighthouse couldn't load the page because Chrome couldn't handle the security configuration of the URL. Example Doesn't detect that page doesn't load due to bad SSL interstitial #1123
   Solution: Fix your server to return a valid, secure TLS certificate.