How-to Fix Duplicate without user selected canonical Issues in Google Search Console

Duplicate without user-selected canonical issue means that CONTENT of the URL Google found on a website is a duplicate of another page, but that URL Google found does NOT tell Google which is the canonical URL.

Check source code to ensure canonical URL is declared.

This is not an error, but is working as intended, because Google does not serve duplicate pages. However, if you think that Google has chosen the wrong URL as canonical, you can explicitly mark the canonical for this page. Alternately, if you think that this page is not a duplicate of the Google-chosen canonical, you should ensure that the content differs substantially between the two pages.

What is URL canonicalization?