Please wait for webmasters response. He will be definitely tell you the reason and add the sub category if found necessary.
Till then keep contributing to AE section and Forum section.
"Winners don't do different things, they just do things differently"
This is because we have not created any sub categories for some Parent categories and therefore it is showing error. We are not sure now what will be the right sub categories to add there that is why it has not been added.