Update install failing macOS Sierra
This solved the problem for me:


Reset the NVRAM using Cmd+Opt+P+R after startup chime.
Boot into safe mode by pressing Shift after the startup chime.
Optionally, take a backup if you don't already have one. I disabled FileVault at this point so I can take backup using a disk image. Not necessary.
Install the updates and let it complete.


Credits: Stuart H