This error occurs when the page has a canonical to a different page.  For example 


The page URL is https://www.mysite.com/en/uk/somepage.html but the canonical tag on the page references a different URL.  In this case it is pointing to the http version.  These are two different pages and they must match.  


<link rel="canonical" href="http://www.mysite.com/en/uk/somepage.html"/>


Common reasons for mismatched canonicals.  

  • Canonical To HTTP or HTTPS version 
  • Canonical to Trailing Slash
  • Canonial to Parameter 

Real example:  

In the example below, Chase Bank has a page for Spanish speakers https://www.chase.com/espanol and they have set a HREFLang to pair it with the main English home page.  When you go to the /espanol page the canonical on that page tells the search engines to ignore the /espanol page and take the global page.  This will create an error and will result in a return tag error for the Spanish version of the page since the search engine will ignore that URL.