A new macOS version exists, no update available from the App Store I recently reinstalled macOS on my MacBook Pro which is now on macOS 10.13.5, which isn't the latest version because I used an old macOS High Sierra installer. However, when I opened the Mac App Store to update to 10.13.6, it didn't show any updates. I thought it was an issue at Apple's side, so I downloaded the macOS 10.13.6 Combo Updater, but that didn't work either. It gave an error: "This volume doesn't meet the requirements for this update". Is my installation corrupted? If not, how can I fix it without another reinstall?

This is a MacBook Pro (Retina, 13-inch, Early 2015). I have enabled FileVault after installing macOS and it is still encrypting (about 13 hours left).
Apparently macOS doesn't allow you to install macOS Updates while FileVault is encrypting, so it blocks every possible way of installing software updates. However, it is still possible to install macOS updates from an external flash drive since there is no encryption going on when you boot from an external device, thus allowing you to install the update. You will need to follow Apple's instructions on how to create and boot from an external macOS Install USB. The only thing you need to do different is before starting the macOS installer, open Disk Utility, select your disk and mount it. You will be prompted for your disk's password since it uses FileVault. After that, simply start the Installer and install macOS onto your main partition. It will not overwrite your existing installation. It will only replace essential system files with fresh ones, thus updating your system.