You actually dont have to use the Node SetStaticMesh when you set it already in AddISMC. I just marked both methods in my screenshot 
Another thing that seems odd to me is, why do you spawn a floorBP and add a new ISMC inside it.
It looks like you’re going to spawn the same Floor and add a new ISMC for the same BaseMesh every time you click Space.
In my project I have 40+ different Meshes that you can place.
I have all of them already stored (as ISMCs) inside an ActorBP that is already placed in the level before begin play.
[spoiler]
So every time I place something it is inside that ActorBP and I don’t have multiple ISMC for the same BaseMesh.
You can download the full source of my project via the link in my signature. Even though it’s quite different it could be helpful