There is one known issue in StaticPress. Isssue of adding new category/tag.

If you want to add new category or tag, for that you have to go to collection of 'Tags' or 'Categories' whatever you want to add and then add new category/tag. Set title of tag/category.

After thet in which post you want that category/tag to be displayed, select that perticular tag/category in that post from dropdown in 'Posts' collection.