AMP Page URL is Not Found (4XX)
This means that the URL in question is an AMP Page URL, which is Not Found (4XX).
Why is this important?
If an AMP Page URL is Not Found (404), search engines will be unable to crawl the page and access the content, which means it will not get indexed or appear in search results.
What does the Hint check?
This Hint will trigger for any AMP URL that returns a 404 HTTP header response.
Examples that trigger this Hint
Consider the AMP URL: https://example.com/page-a/amp/
The Hint would trigger for this URL if it had a 404 (Not Found) header response:
How do you resolve this issue?
This Hint is marked 'Critical' as it represents a fundamentally breaking issue, which may have a serious adverse impact upon organic search traffic. It is strongly recommended that Critical issues are dealt with as a matter of high priority.
AMP URLs need to be accessible in order to be indexed. In this case, either the page is missing/removed, or the <link rel="amphtml"> tag on the non-AMP page is pointing to the wrong URL.
To rectify, you would need to ensure that the <link> tag is pointing to the correct (i.e. equivalent) AMP page, and ensure that this page is fully accessible to search engines and users alike, returning a 200 HTTP response.