Canonical is empty issue means that URLs have empty rel = ”canonical” tag.
The importance of the issue
Canonical is used for setting the main URL for a search engine, when there are other URLs with duplicated content. If the Canonical attribute is not set, it will be ignored by a search engine, thus leading to indexation of an unsuitable content.
How to check the issue
Using any browser is enough to check the issue. Open the source code of the flawed page To do this, click the right mouse button at any spot of the page and choose “browse the code” option, or apply an online tool https://codebeautify.org/source-code-viewer
Analyze not only canonical being empty but the entire site!
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Find the link tag with the rel = ”canonical” attribute.
How to fix this issue
If the canonical generation takes place automatically, it is necessary to review the algorithm. The issue is mostly caused by manual settings. In this case, you need to set a correct value of a href attribute for rel=canonical in a specific page configuration.