I’m trying to add viewport layers by copying and pasting the existing viewport, but Layout’s SketchUp Model panel is greyed out… for the existing viewport and the one I’m trying to insert. What am I doing wrong?
It sounds to me as if you aren’t selecting a SketchUp viewport. Several possibilities: You’ve selected a group–maybe a viewport with a clipping mask. The viewport is on a locked layer and you’re selecting something that overlays the viewport. You’ve previously exploded the viewport so you aren’t actually selecting a SketchUp viewport.
All guesses. I could tell you with 100% certainty if you’d share the LO file.