If you notice you can’t install new macOS updates on a Mac, it could be that it’s still in the process of FileVault encrypting.
For example, here’s a machine that’s on macOS 10.13.4.
softwareupdate can’t find any updates.
And even if you try to manually install the 10.13.6 combo update, you get macOS High Sierra 10.13.6 Update can’t be installed on this disk. This volume does not meet the requirements for this update.
And, yup—lo and behold! The FileVault encryption is still in progress. Once that’s done, the 10.13.6 update should install just fine.