amboutwe on "[Plugin: Yoast SEO] Canonical in categories"

Likely good to check your permalinks, redirects, slug names, etc.

The link you provided above redirects to another website, so obviously that canonical link will never work as "that link" as you put in your post here.

Also note you cannot have a tag, category, page, and post with same "name" -- each item should be unique, otherwise it causes problems.

We are not having this issue on 20 sites running both free and pro versions of plugin, so not sure what the issue might be.