[SM5] (Node LandscapeLayerBlend) Arithmetic between types float3 and float4 are undefined
I’m very new to unreal so if this is a very beginner question, so sorry.
You’re trying to blend RGB ( 3 ) with RGBA ( 4 ), not possible. Also, why? 
Where you get the height from really depends on how the person who made the textures set things up. But typically, it would either be on the red channel, or in a separate texture.
A lot of textures don’t even have one…