Extrude shape that has been draped onto a Sandbox created terrain

Hi all,

I’m a relative beginner to SketchUp. I have Pro and have just created terrain using the Sandbox and have draped my garden design plan over that terrain. So far so good. I need to extrude steps into that terrain for an elevation but it says that I cannot extrude when I select any of teh area that I copied onto the terrain. Pic attached. Do I need to do something else?

With Fredo6 JointPushPull