What is the issue?
The canonical tag indicates which page version Google should index, and it's always a best practice to include a canonical tag even if it's self referencing. We flag an invalid canonical when the page is missing the tag, or if you are referencing a non-indexable page. This should be fixed as soon as possible to ensure search engines know the focus page to index and serve in search results.
How to fix
If a page is canonicalized, it shouldn't receive any incoming organic sessions. A broken canonical can still result in the page being indexed and visited via search results. Use Google Analytics data to see which broken canonical receives the most traffic, and ensure you fix this first. Learn more in this article.