Your problem is that you created your NBI from sierra. I have not added support yet. Apple changed the outputs on Sierra for diskutil, which drives me mad. This should stay the same b/w versions or else scripting will break, as is the case here. If you want to deploy sierra I believe you could do it, but you must use an el cap nbi. Although I haven't tested deploying sierra with el cap nbi.
I wanted to get sierra nbi support in 1.2.0 but I discovered a bug with Sierra that is preventing the image from saving to smb share correctly. I have since opened a case with Apple and am waiting for a resolution.