What is the canonical Issue?
A canonical URL is the URL of the page that Google thinks is most representative from a set of duplicate pages on your site.
Canonical issues most commonly occur when a website has more than one URL that displays similar or identical content. They’re often the result of not having proper redirects in place, though they can also be caused by search parameters on e-commerce sites and by syndicating or publishing content on multiple sites.
That means the canonical URL element informs Google and other search engines to crawl a website, and what URL to index that specific page’s content under.
Canonical is where any other website or page URL is different but the content of the page is the same. It is also called duplicate content or near to duplicate or closely duplicate content.
If you have the same or similar content available under different URLs, you can use canonical tags to specify which version is the main one and thus, should be indexed. Imagine crawler comes to your website or page and sees that there is same content on two pages, it may be confusing for the crawler, while indexing. Hence in such a case, you need to apply canonical tags on the page to make them aware, that this duplication is not an error but by design.
If you have AMP and nonAMP versions of a page. Crawler does not know which page he has to index and which not to.
Having separate printable versions of pages, gives priority of which content you want to index. All the hints which we are giving to Crawler is what we can do through canonical tags. The URL of the original page is our master page and where we set it in the page is called a master copy duplicate page. It is recommended to use only one canonical tag per page. The canonical tag is a page-level meta tag that is placed in the HTML header of your webpage. You have to add canonical tag on WordPress if you are using it, you can get an advanced yoast plugin in WordPress and set your canonical tag there. Just write a simple URL there.