"Panel: A basic container panel" is what you can add to the interface to add more panels.
If you find "the parent has no free slots for the child", it might be due to that you have default "Border layout"
and you have used all five possible positions(left,right,center,up,down) to add new panels.

You can re-parent one of them as child of another panel.

I do not understand what you really want to do. So I do not know how to help. Sorry.