However, if you are referring to this http://mhasegawa.com/about-fortleft/ then note it’s a Page and not a Post. http://en.support.wordpress.com/post-vs-page/ Pages cannot have any categories or tags assigned to them. They are only for static content that rarely changes. Only Posts can have categories and tags assigned to them. Also note that there is not automatic way to move the content in a page into a post. It’s a copy and paste content into a new post and delete the page task.