Non-canonical URLs in the sitemap mislead search engines. It happens when a page’s URL doesn’t match a canonical URL. As a result, the robots are indexing pages that have different addresses from their canonical versions. To avoid these problems, one should always put a tag on a preferred URL. For instance, if the same page can be accessed with and without WWW, you should tag the one you want search engines to index.
How Do I Fix Non-Canonical URL In My Sitemap?