macOS Sierra update can't be installed on this disk.This volume does not meet the requirements for this update
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.