macOS Sierra update can't be installed on this disk.This volume does not meet the requirements for this update I have been updating the macOS from 10.12.4 to 10.12.6 . For this I have downloaded the macOS dmg file from the Apple website. But when I go on for installing the macOS then I see this error. 



I have around 100GB of space but its still not updating the OS. What should I do about it. I need to install "Xcode" from the App Store but when I go on for installing it gives me the error that it needs the OS update to be 10.12.6 How can I fix this up?

The version I am working on is 10.12.4
I just bought a reconditioned MacBook Air running macOS High Sierra 10.13.4 and as I wanted to upgrade to macOS High Sierra 10.13.6, I ran into the same problem. I tried everything and was about to give up, until I stumbled the following page:


https://technology.siprep.org/waiting-for-filevault-encryption-to-finish-to-install-macos-updates/


and realized that FileVault was still encrypting my hard disk and that was blocking any macOS update.