Thanks for your answer. Yes I am running the latest version of Greatmag and I use a childtheme.
It seems that the error appears only for the articles with comments.
For exemple this page “https://geonautrices.com/bilan-gr-tour-du-mont-blanc/” has 15 errors of missing: fn. The errors are related to the type of data “hcard” in the GSC, source “microformats.org”. When I clicked on this line, I have the details of all articles’ links which have errors and when I clicked on an article on the list, I have the details of the data detected “hcard” with “itemtype” and “photo” for each hcard. There is as much hcard data as comments in the article.
And that’s it. I don’t know if these informations could help you to see the problem.
I tried to check the tool for testing structured data of GSC as well but there isn’t errors detected…
Thank you for your help and your time, I appreciate it ! (and happy new year ! )