Not applicable to this computer?

I had the same problem with that particular KB update to only find that one of my computers (laptop) wasn't running the Fall Creators Update. I had to first manually install the Fall Creators Update and then check for updates. I have no idea why the automatic updates didn't upgrade the laptop to the latest build; but when I found that was the problem, all updates afterwards installed correctly.

Skofab said:

I have downloaded KB4040724, (both versions) and attempted to install, but without success.
In each case the following message occurs....
This is not applicable to this computer (or words to that effect)

This message can occur if the update you try to install is for a different version of Windows 10 from the one the PC is running. KB4040724 is for version 1703, os build 15063. What version of Windows 10 do the two PCs have? Run Winver to find out. Maybe they updated to Fall Creators Update, version 1709, os build 16299 without you noticing.

Anyway, why are you trying to install September's update? It's not the latest one. A new cumulative update was issued in October which included all September's fixes (then some more - and November's one is due out in five days). The 'not applicable' message may also mean they are running 1703, but already have a newer update than the one you are trying to install.

I'd run Winver on all your machines, just to be sure they're up to date. They are only up to date if they are running version 1703, os build 15063.674 (for the Creators Update) or version 1709, os build 16299.19 (Fall Creators Update).