The error you are receiving indicates that eBay has either changed the category number (thus invalidating the original category number), has added sub-categories to it (thus it is no longer a "leaf" category), or has removed the category altogether.
To eliminate the error, please edit your listing, scroll to the category section, and click the "Edit Categories" button to reselect your category. Be sure to choose the category from the menu selector, and save your changes after reselecting your category.
After taking the above steps, you should have no further trouble listing your item.
