If Joseph's correct and helpful suggestion does not produce results, then you'll need to attempt to reinstall macOS.
In doing the update to high Sierra my Mac air entered in a loop where it says that there are problems on installing the update so to exit and try again, the problem is that I can exit because if I restart the computer it’ll try again to install the update. Any suggestions??
2 replies
Boot the computer up in safe mode - Apple Support.
MacBook Air doesn’t update