Canonical points to a URL that is Not Found 404
This means that the URL in question has a canonical element whereby the canonical URL itself was Not Found (404).
Why is this important?
The point of a canonical is to explicitly and unambiguously indicate a preferred URL. However in this situation, since a 404 page is not indexable, the canonical instructions are sending conflicting messages to the search engines - which typically means they will ignore the canonicals entirely and try to make their own decisions about what 'the canonical URL' should be.
The net result is that it could lead to incorrect URLs being indexed, resulting in duplicate content issues.
What does the Hint check?
This Hint will trigger for any internal URL which contains a canonical link element that uses a 404 URL as the canonical URL.
Examples that trigger this Hint:
Consider the URL: https://example.com/page-a
The Hint would trigger for this URL if it had a canonical URL pointing at another internal page,
where this canonical URL https://example/com/page-b responds with a 404:
How do you resolve this issue?
This is a scenario where the canonical configuration is completely broken. You will first need to establish if Page B should actually be a 404, or if the URL could and should be resurrected. If you can resurrect Page B, then you can leave Page A alone.
Assuming that Page B indeed should remain as a 404, then you will need to adjust the canonical URL for Page A - so that it is either self-referential or pointing to another URL.
Issues like this will typically need to be resolved on a case by case basis.