can't copy and paste

[Ofnuts]

This is because there is no point in the selection that falls in the layer boundary (or form the other point of view, no pixel in the layer under the selection mask), and by default "Copy" is "Copy the part of the active layer which is in the selection".

   

So either you want to make the proper layer active, or you use Edit > Copy visible (Ctrl-Shift-C) so that Gimp copies whatever falls in the selection.