Change the Alpha input of your Lerp to one of the output channels of that Texture Sample rather than the RGB output. RGB output (the black square) is all 3 channels R, G, and B in one. EDIT: You need to feed the Lerp's alpha input a single channel such as Red, Green, Blue, or Alpha channel output from the Texture Sample.
It's my first time working with the material system in UDK and I've stumbled on this weird error. I've been trying to get the glossiness to work as it does in Max, and from what I read online, the best way is to plug a Lerp node into SpecPower with min and max constant node values plugged into A and B and the glossiness…