I quit and restarted choosing start up disk Macintosh MAC OS 10.13.6 but the problem keeps repeating every time I use the mac.
Also constantly getting message Safari Bookmarks sync, agent quit unexpectedly.
iMac 21.5", macOS 10.13
Have you downloaded the installer and you are in the process of trying to install the upgrade? Try booting into the Safe Mode to see if it will install. If that doesn't work, please provide more details about what you did to get where you are seeing this message.
Mac OS cannot be installed
