The macOS installation couldn't be completed - Solution
You just did a macOS update, drank coffee in between and now the Mac shows the following error message:
****The macOS installation couldn't be completed****?
So the situation is that the Mac stumbled over something and couldn't complete the planned software update. If you look through the error messages in the background, you might see the line: "The main OS Installer distribution did not load". Which of course would explain why nothing can be installed if the installer is not found.
If you simply restart now, you will end up in this error message window again. A new installation or even a restore of a Time Machine backup is usually not necessary, the solution is much simpler.
Just try the following simple trick: With the options offered, select "Restart" and hold down the alt key while the Mac reboots. The Mac should then display all boot drives. Select your main drive (SSD in this example).
Now the Mac should simply ask for your login data for your profile again and boot normally, as before the failed update. You will now also find the wanted update in pending updates as it did not install. To be on the safe side, simply restart once more and try the installation again.
This error apparently occurred with us because the system is on the SSD, but the user folder is on a hard disk. After the installation the Mac booted normally, but we didn't log in and the Mac went into standby. The next time we started, we got the error message. It can also be the case if the computer lay around for a long time and then wants to make an update directly that it gets stuck.
But before you play back (your hopefully existing) backup, just try the trick with the alt key. And, as always, don't freak out when the Mac beeps.