Because of uniqueness, you can't copy CAD Docs from one product to another. The identity of the CAD Doc (numer/name/filename) must be unique across all products and libraries. The same is not true for projects--you can have CAD Docs with the same identity in multiple projects What you are trying to do is not supported.
You can use the Move or Cut/Paste actions to move from one product to another, but they cannot be copied..