Hi,
When you arrive on your backend products listing, it displays by default all the products from the main category as well as all the products of the sub categories below the main category.
So you might have products from different categories displayed on the same listing. Since the ordering of products is tied to the category they are in, it is not possible to order products from different categories together. That's why you get that message.
When you change the dropdown to "direct sub elements", you now only see the products directly linked to the main category.
If you don't see any products then, it simply means that you don't have any products directly linked to the main category.
On the top left part of the listing, you have an icon "<>" you can click on. This will develop the categories explorer on the left of the listing. That way, you can change the current category. Select a category with products there, and you'll now see the products of that category, and you'll be able to order them.