I had received this message before so I checked the Google Console manager to see how many pages were not indexed due to this condition. (This is a geneaology website with more than 150,000 individuals so there were a lot of pages)
Before installing the mod, I checked a link that could not be indexed. Notice the referring page links to a different Google selected canonical.
This means the referring page is not indexed and does not appear in Google search results. The Google selected canonical page does appear in search results.
After installing the mod, the page can be indexed as shown below below.
Adding a canonical tag does not guarantee a search engine will index all of your website. Canonical tags reduce can duplicate issues and ensure links are accurate and connect to the correct person and not someone else.
The original mod was written so the canonical link was the same as the agent mode used by the bot or crawler (standard or mobile).
After numerous tests with Googlebot is was determined this was detrimental to pages being indexed as shown below.
The latest revision uses a generic canonical link that is not dependent on the crawler or bot scanning mode.
TNG WIKI Download Link for v13 and v14
Revisions:
29 May 2024 - v13.1.2.2.zip - Removed site version from canonical link
09 May 2024 - v13.1.2.1a.zip - First Public Release