MacOS 10.14.1. MacBook Air,

Do you have sufficient storage remaining on the Air for Software Update to successfully install 10.14.6 and subsequent browser and security updates? The timeframe from 10.14.1 to 10.14.6 is nearly 9 months of missed updates.

This one , I am not 100% sure it will work in Mojave. and stand to be corrected - ok.

Restart in Recovery Mode ( Command + r ) and choose Reinstall macOS.

The theory is, it will pull down the latest version of Mojave 10.14.6

Try updating while in Safe Mode.

How to use safe mode on your Mac - Apple Support