What triggers this issue?
This issue reports all canonicalized URLs listed in the website’s sitemap files.
Why is it important?
Sitemap files of a website must only list the pages of your website you want search engines to crawl and index. Besides, Google uses sitemaps data as one of the factors when selecting canonical URLs.
Non-canonical pages in sitemaps send a misleading signal to search engines, instructing them to index the URLs that declare a different URL as their canonical version.
This can result in search engines ignoring your sitemaps. In this case, you might end up with some indexability issues on your website.
How to fix it?
Your sitemap files must only include canonical URLs.
Remove the canonicalized URLs your sitemaps.