Hello Andymallins,
There are a variety of reasons Mac users may see the “Download failed” error when trying to download and install a particular MacOS software update, and we’ll go through some troubleshooting steps to help resolve the issue here as well as offer an alternate solution to download macOS installers even if the System Preference panel for Software Update is repeatedly showing failure errors.
Troubleshooting macOS “Download failed, An error occurred while installing the selected updates.” Errors
- Reboot the Mac
- Sometimes simply rebooting the Mac and trying again can resolve the failed installation error.
- Wait a bit
- Users may encounter failures to download and install macOS updates when Apple servers are overloaded, therefore sometimes simply waiting a bit can be helpful.
- Make sure the Mac is online and functionally connected to the internet
- Key here is making sure, Internet is working (This may be 1 of the reason of failure)
- It can also be DNS issue in the local mac or local router level
- I am assuming, his internet is working correctly on iMac when he tried for the upgrade
- Alternate Solution - Try downloading the macOS installer directly
- This bypasses the Software Update system preference panel on the Mac
- App Store Link
Hope this will help