'The macOS installation couldn't be completed' Issue

Mac users usually encounter the 'The macOS installation couldn't be completed' error after updating to a newer macOS version. The system first downloads the newly released version, before proceeding to install it on the Mac. However, after performing the required restart, some users may be greeted by an alert stating that 'The macOS installation couldn't be completed.' They are given four options to choose from - 'Shut Down,' 'Restart,' 'Save Log' and 'View Log.' Unfortunately, shut down and restart would not help with the system arriving at the same alert window every time it is started in what appears to be an endless loop. Users still have some options at their disposal, though, so take a look at the suggestions outlined below.

Boot the Mac into Safe Mode

Safe mode starts the system with only the bare minimum of functions and services eliminating the risk of interference from additional applications or extensions. At the same time, it also attempts to repair potential disk problems via the Disk Utility's First Aid service. The cache and other temporary files that may have piled on the system will be deleted. As such, Safe Mode is a good starting point when dealing with the 'The macOS installation couldn't be completed' issue. The specific process for starting Safe Mode differs, depending on whether the user's Mac is equipped with an Intel or Apple (M1) CPU. 

Safe Mode for Intel-based Macs:

1. Shut down the Mac and wait for at least 10 seconds.
2. Boot up the system while pressing and holding the Shift key on your keyboard.
3. When you see the login window, let go of the Shift key.
4. Log in by entering your user credentials. 
5. Finally, you should see 'Safe Boot' displayed in the top corner of the login window.

Safe Mode on M1 Macs:

1. Shut down the device.
2. Press and hold the Power button.
3. When the startup options appear on the screen, let go of the Power button.
4. Select the startup disk.
5. Press and hold the Shift key on your keyboard before pressing the 'Continue in Safe Mode' option. Release the Shift key afterward.
6. Enter your login credentials.

Boot the Mac in Recovery Mode

If Safe Mode failed to address the issue, then the next step is to try booting the system in Recovery mode and running the First Aid service. As a last resort, you may need to reinstall the entire macOS system from the Recovery partition. 

1. Shut down the device.
2. Press and hold Command+R on your keyboard and then start the Mac.
3. From the available macOS utilities, select Disk Utility.
4. Now, select First Aid and confirm your decision to run it.
5. Wait for the service to scan your disk and then attempt to fix any potential problems.
6. When it is finished, restart the system.

If the 'The macOS installation couldn't be completed' error persists, then it is time to reinstall the macOS. Boot into Recovery once more but this time choose 'Reinstall macOS.' Wait for the process to complete and then try installing the latest update once more.