@Whereditgo Wall framing can only live inside of the wall, and when you copy+click to paste, it isn't trying to paste it into the wall object, meaning you're met with that message.
Instead to copy/paste wall framing, in any view, you're going to select the stud, click Copy, and then use the Move edit handle to move a copy of the stud.
I have created a short gif that demonstrates this process in a cross section view, but this can be used in camera view or in plan view.
If you're curious on how to create a cross section framing view, please see this Knowledge Base article: https://www.homedesignersoftware.com/support/article/KB-01188/creating-an-elevation-view-of-wall-framing-in-home-designer-pro.html
ChiefTylerL's post in Not able to add (copy/paste) wall studs. was marked as the answer