AMP Page URL has canonical URL which is Not Found (4XX)
This means that the URL in question is an AMP Page URL, and contains a canonical tag which returned Not Found (4XX).
Why is this important?
AMP HTML documents are required to have a canonical to the non-AMP equivalent (or a self-reference, if no equivalent exists). If this canonical is pointing to a URL that is Not Found (404), search engines will be unable to access the page.
The canonical tag is a mandatory element for AMP pages to be considered valid, and the canonical tag is supposed to point back at the original 'non-AMP' version of the page. If this page is inaccessible then search engines cannot crawl it, which makes the AMP pages more difficult to discover, and sends conflicting and confusing signals to search engines - increasing the chances that the AMP page will not show up in search results.
What does the Hint check?
This Hint will trigger for any AMP Page URL which has a canonical tag pointing to a URL that returned Not Found (4XX) when Sitebulb attempted to crawl it.
Examples that trigger this Hint
Consider the AMP Page URL: https://example.com/amp/page-a/
The Hint would trigger for this URL if it included a canonical URL,
where this canonical URL had a 404 (Not Found) header response:
Similarly, URLs with other 'not found' statuses will trigger the Hint (i.e. any other 4XX 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.
Assuming the canonical is pointing at the correct URL, you would need to look into why the URL is no longer accessible (presumably it was accessible in the past). If the page has been removed or deleted, you would need to republish the page on the same URL, and ensure it returns a 200 status.
If the canonical is pointing to the incorrect URL, you would need to fix the canonical on the AMP page so that it points back to the (correct) non-AMP version of the page, and ensure that the non-AMP version of the page has a self-referencing canonical. It should look like this:
For the URL: https://example.com/pages/page-a/
This page defines a self-reference canonical and an AMP page:
Then the AMP page has a canonical pointing back at https://example.com/pages/page-a/