I have been fighting this issue for quite a long time, since I bought a new Mac mini early this year and restored the system with my TimeMachine backup, every time when there is a Mojave update comes out, I always got this “update failed” warning.
And my first idea is “What do you mean? Come on!”.
Then I googled and tried like everything:
Turned off System Integrity Security in safe mode
Install from a downloaded Combo Update installer
And it ended up with No Luck!
Then I tried another direction,
Backup, Install new MacOS, Restore from the backup
Sure this is working, but that would be pretty annoying and stupid whenever there is a new MacOS update to install, you have to go through this wasting time process! (Or you just ignore the updates).
Then just previously after Apple released MacOS 10.14.5 update (and yes I went through again the above painful process to install the update), it was found that Chrome can easily crash MacOS Mojave 10.14.5.
“Chrome keeps crashing on MacOS Mojave 10.14.5”
Chrome keeps crashing on MacOS Mojave 10.14.5 - EXC_BAD_ACCESS (Code Signature Invalid) - Google…
Process: Google Chrome  Path: /Applications/Google Chrome.app/Contents/MacOS/Google Chrome Identifier…
Wow, that sounds bad, right?
However it turns out to be a chance for me to hit into an alternative to this “unable to verify software update” problem!
Good job Google!
Because I just did a backup last week for updating 10.14.5, so there is not so much to do another backup during this week. I decide to directly boot into Recovery Mode (Start your Mac, press and hold Command+R), and reinstall MacOS from there. (Please reserve about 30+ GB disk space)
Guess what, after I reinstalled the macOS, it actually completes the macOS updated to 10.14.6!!!
I hope I can come out this alternative earlier, but still it is ridiculous Apple haven’t provide any fix for that yet.
“Unable to verify MacOS”
"Unable to verify MacOS" - Apple Community
Hey, DukeOliver. I see that you're trying to update 2018 MacBook Pro, but you keep getting a message that the macOS…
“Unable to verify macOS” error while updating to Mojave
"Unable to verify macOS" error wh... - Apple Community
Hi, platonish, Thanks for visiting the Apple Support Communities. I understand that when trying to update your 2018…
Unable to verify software update
Unable to verify software update - Apple Community
I am currently running macOS Mojave version 10.14.3. and trying to install the available update in Software Updates…
I’m not sure if my approach works for others, maybe Apple’s team is more focusing on the new macOS Catalina, which could eventually eliminate this issue.