After Sierra install, message appears saying install failed, Hold down D for Diagnostics
The error means that the file /sbin/launchd does not exist. The file is an important system component of macOS which is responsible for starting all other programs. Basically without it you cannot boot your system.

Normally this would mean that something has very seriously failed during the installation of macOS.

I would suggest reinstalling macOS while paying very close attention to any error messages that popup during installation.

If problem persists, consider the possibility of a hardware error such as a disk failure.