Failed installation of macOS Sierra Update 10.12.6

in system preferences > App Store verify that these settings are checked .

And in App Store , click on check for unfinished downloads .

If these didn't work App Store plist from user library has to be cleared later and some more things from system library .

First try this option and see if it works .

I am

Try installing the combo update while in safe mode. You may have something installed preventing the update.

Use safe mode to isolate issues with your Mac - Apple Support

Sorry, I did not notice you had already tried the combo. I should have read more carefully.

There is probably something peculiar to your installations that is causing this. It is certainly not a common occurrence.

I suggest you run Etrecheck on one of the affected machines and post its report here. This may gives a clue as to what may be happening.

Try downloading and installing the Combo Update.

Download macOS Sierra 10.12.6 Combo Update

Try re-indexing Spotlight.

Spotlight – Re-index

The App Store uses it to know why is installed on the computer.

You're welcome.