You have duplicate posts so I'll only answer this one. It is very likely that you are using ray-traced rendering in the composition settings. We would know if you had included a full screenshot. Ray-traced rendering does not support blend modes on 3-D layers.
You're right, Rick. Layers 2 and 6 both have Geometry Options so they are 3D layers in a Ray-traced comp. By the looks of things you're wanting to cast a shadow on the ground plane and set it to multiply? Well you won;t be able to do that. I can't remember exactly if the raytraced layers had this feature, but is there an option in material settings to accept shadows only? That will give you what you want.