Macbook air 2018, current macOs is Mojave 10.14.6. I havent used it for about a month and it has been in sleep mode. I was prompted to restart computer since updates have been applied. While updating, there was a black screen with a pop up stating that message and saying you can try updating again in system preferences after restarting.
Perhaps the update files were already ‘outdated’ by the time you restarted the Mac. Or the system time was not accurate anymore, due to the Mac sleeping for so long.
I am not familiar with the intricacies of the update process, but the error message suggests that there is a code signature problem with one of the installer packages that macOS tried to install. The code signature might have been invalidated by then or its validity couldn’t be verified.
Retrying the update, i. e. by downloading new copies of the packages, and making sure that the system time is set correctly could resolve the problem in that case.