I should clarify that I can remove other elements, just not this one element.
Yes, I've selected the terrain from the dropdown or by clicking on it.
I'm not sure about the unlock icon on the heads-up toolbar. What do you mean by that?
And to clarify further, I wound up deleting that terrain and starting over with it because this terrain model was fairly uncomplicated, so I can't really submit anything to troubleshoot. But I really don't like that as a my only solution or workaround.
Now that I think about it, this is probably another outcropping of another issue I've had before where I couldn't remove other elements from other terrains. The disconnect this time was perhaps simply that this element was set up as a boundary.