The noindex robots–meta value discourages search engine robots from indexing your page in the search engine.

When no posts are attached to a category, it’ll be discouraged from being indexed. WordPress can send out a 404 signal; but, this isn’t always the case.

So, to improve your website’s SEO by removing thin content, empty categories automatically have the noindex robots–meta value set.

To remove this discouragement, you should assign posts to it.