Canonical is a relative URL issue means that the URL has a canonical element which uses a relative URL.
Why is this important?
Google state that one of the most common problems 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.
How to fix?
It is strongly recommended that all pages 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.
Detect not only canonical being a relative URL but also other kind of technical issue on your site!
Crawl your site and find out all kind of issues that can hart your users or your website SEO.