A new macOS version exists, no update available from the App Store
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.