Apple - 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.

In my experience, downloading and installing the Combo Update sometimes works when the Delta update fails. Please edit your question to indicate the specific Mac model you are working with.