Canonical issue is of duplicate web pages, if by possibility your site 2 page same content and different urls but you can't redirect it so their using canonical tag. Canonical tag using in head section on your site.
Canonical issue is issue of duplicate pages, if your site 2 page have the same content but different urls and you can't redirect it using canonical tag. Canonical tag used in head section on your site.
Fixing canonical issues means that you do a canonical redirect so that if someone (search engine or human) tries to go to the wrong version of your URL, they will be redirected to the correct version.
If your site runs on a Windows server, a canonical issue is easy enough to fix with a 301 redirect.