I initially ran in to the same problem after downloading Sierra from the App Store, a reboot of the VM resolved the problem.
I have now done fresh builds and installed El Capitan and Sierra from the iso created with shela's new script.
CECI.tool and CSI.tool does not detach OS X Install ESD.
'hdiutil attach' command cannot attach the same image twice, so my script reports error.
This is a defect of CECI.tool and CSI.tool.