When you receive the 4XX error code for canonical links on your website, you need to understand this: The page that the link points to may not accessible to the browser –for various reasons. Therefore, indexing cannot be performed. Failure to perform indexing may mean that increasing the ranking is just a dream. So, in order to optimize your SEO score, you need to fix the canonical unsuccessful status code problem as soon as possible.
So what should be done to solve this problem?
This error indicates that there was a problem receiving requests between the server and your browser. Check for URL errors.
Canonical links fail on pages that can be viewed by users logged in to your page, but where everyone else is considered unauthorized. Then make the page public or remove the canonical link.
You may get this error if the web page you marked with the canonical link is banned for a certain audience. There are two things you have to do: If you insist on having the page forbidden, remove the canonical link from the page. If you wish, make the page accessible to everyone.
The most common problem for canonical links and other web page errors is undoubtedly the 404 problems. You can get this warning when the URL of the relevant page changes or the page is deleted. So you need to check the URL.
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