What triggers this issue?
This issue reports the URLs in the website’s sitemap files that have no incoming internal links from any other pages on the website.
Why is it important?
Similar or duplicate pages of a website must specify the canonical URL to instruct search engines to show the most authoritative (canonical) version of the page in search results.
If a URL has no incoming internal links, there is no way for people to access it from your website. Besides, such URL receives no link equity (or "link juice") via internal links, which does not help this page to rank.
Most likely, this issue results from some kind of misconfiguration on the website.
How to fix it?
Check your website navigation and link architecture and make all canonical URLs easily accessible. Make sure that the pages you want to rank on search receive a good number of internal links.