What triggers this issue?
This issue reports pages that have a canonical link pointing to a redirecting URL.
Example:
For https://help.ahrefs.com/en/
<head>
...
<link rel="canonical" href="https://help.ahrefs.com/en"/>
...
</head>
https://help.ahrefs.com/en declared as canonical, returns the 301 HTTP status code and redirects to https://help.ahrefs.com/en/.
Why is it important?
Canonical links are used to solve duplicate content issues. If you have several pages with the same or similar content, you need to pick the most authoritative (canonical) version.
Redirecting URLs declared as canonicals send a misleading signal to search engines, instructing them to index the URL that does not return the 200 (OK) response code, but redirects to another URL instead.
The search engines might ignore this instruction and choose a different canonical URL based on other factors.
How to fix it?
Canonicals must be live URLs that return the 200 (OK) response code.
Replace the redirecting URL declared as canonical on the reported page with the live 200 (OK) page version you want to be indexed in the search results.
The canonical page can be specified by:
using a rel=canonical <link> tag in the code of a duplicate page or
using a rel=canonical header in your page HTTP response.