my macbook air 2017. I had the same problem so I returned to the initial system and the computer apparently returned to normal. It only works if you already have macOS High Sierra installed. Shut down your Mac and turn it on while pressing CMD + R. This will start your Mac in Recovery Mode. Access Disk Utility, select on the right tab “Macintosh HD”, the location where OS X is installed, choose the Delete tab and the Delete button there. Close Disk Utility. And choose Install macOS High Sierra. It will be downloaded and the installation will proceed normally.