Canonical is missing issue means that there are pages that have no canonical tag.
What Is a Canonical URL?
A canonical URL is the version of a webpage chosen by search engines like Google as the main version when there are duplicates. This selected URL is prioritized to avoid showing repetitive content that does not provide unique value in search results.
Checkout this blog for more info: https://www.semrush.com/blog/canonical-url-guide/
How to fix this?
add below tag in <head> Section of your page.
<link rel="canonical" href="https://example.com/preferred-url-here/" />