Old shader model vs new shader model conflict? Try enabling the old shaders in Houdini, maybe then it will work. I recall several posts over the recent weeks explaining how to do it.
I believe that this is because Houdini is Mantra centric by default. You need to go to Edit > Preferences > Rendering to reveal other renderers or focus completely on one of them.
Robert Magee Senior Product Marketing Manager SideFX