Canonical is a relative URL
This means that the URL in question has a canonical element which uses a relative URL.
Why is this important?
Google state that one of the most common issues they see with canonicals come from the use of relative URLs.
The link tag accepts relative URLs, so it is actually valid HTML. However using them in canonicals can lead to other issues, such as the base URL being incorrectly defined, which would completely ruin your canonical set up.
The point of a canonical is to state precisely which URL is the preference, without ambiguity. This is best achieved by using absolute URLs for your canonical URLs.
What does the Hint check?
This Hint will trigger for any internal URL which contains a canonical link element that uses a relative 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 using a relative URL;
Canonical link in the <head> to a relative URL
OR HTTP Header canonical link to a relative URL
How do you resolve this issue?
It is strongly recommended that all URLs are updated to use absolute URLs in canonicals. Often, this type of issue is controlled by particular rules or page templates, so it might be possible to solve this issue for all or lots of pages at once, with a few changes to the governing rules/templates.