The vast majority of Macs update to mac OS Sierra without any adverse incident.
Sometimes when users attempt to download mac OS Sierra from the Mac App Store they will confront a “An error has occurred” red message and the download stops.
Typically this means the internet connection was disrupted, or the file itself somehow was interfered with during the transfer.
You will need to delete the “Install mac OS Sierra.app” and download it again from the Mac App Store.
You can also try disconnecting all external devices, which has been reported to help some users shut down Mac OS Sierra.