I have a MacBook Air and when I tried to install High Sierra, I was unable to do so because I received the message that I needed to turn FileVault off. When I went to FileVault it showed that it is decrypting but does not show the time remaining. I let it run for 8 days with no change. I have tried turning it off in Terminal but I get the message that FileVault is off. I have tried rebooting in recovery made but this did not help.