oh... The Construct's Background material...
You can't add another layer to the Background material zone of PP11's The Construct prop.
You can add more layers to its Ground material zone, though.
Interesting discovery for me.
I wonder why it is so...
@ibr_remote Hmmm, that is interesting. Maybe it's only got one layer because it's assumed folks would be using HDRi (or similar) images, so they don't need other layers?
Just a guess on my part.