Soft 404 errors happen when a web crawler tries to index a webpage and there is no content in that url. The problem happens if the web server does not return a 404 error code (and, instead a 200 return code). This is a problem for web crawlers like google because it makes it more difficult to structure the information from the web.
In the case of wordpress, this can happen with the “uncategorized” category. When there are no posts in this category, trying to access this category would not return a 404 error but also there is no content.
One way to solve this problem is by creating at least one post (like this one) in this category. The other alternative would be to delete the “uncategorized” category, but this can lead to unexpected behaviour, since this is the default category for new posts.