Canonical is not an absolute URL

What this means

The rel=canonical value is relative or malformed rather than a fully-qualified http(s) URL. Google resolves relative canonicals against the page, so the tag usually still works — but it silently resolves against the wrong base whenever the page is served under a different host, protocol or path, which is exactly how duplicate-content bugs start.

How to fix it

Write the canonical as an absolute URL including scheme and host, e.g. https://example.com/path.

Fix effort

This check needs review: the change can be drafted for you, but a person has to approve the wording or choose the target before it ships.

Authoritative source

https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls