Error 8705 target layer cannot be modified

Hi all,
When I am working around with layers, I am getting this error message. I do not understand the reasons for the error. The error triggered when I try to copy layers in the clipboard and it is not allowing me to work with the layers. Does anyone know how to fix this problem? Please help. Thanks.

Adobe Illustrator
Error 8705: Target layer cannot be modified
Line; 531
-> Var g = doc.groupItems.add();
