The PSO creation in DX12 fails due to color data not being fully written in vertex stage. This is a request for shader compiler to give better debug output for this. If an output is declared in vertex shader and is read on the pixel shader all the values must be written into with no values being left as undefined. So a float4 needs to have all 4 components filled. Shader compiler could theoretically do it it automatically but currently it does not.
After manually setting all the unfulfilled output values in the vertex shader it works.