Can't install the software.

Try booting into Safe Mode to see if it will install the update. Safe Mode prevents third party software from automatically launching during boot & login. If this works, then you have some third party software installed which is interfering with the normal operation of macOS. The usual culprits are anti-virus apps, cleaning apps, and third party software. If you have these types of apps installed, then I highly recommend you uninstall them by following the developer's instructions. These types of apps are not needed on a Mac and usually cause more problems than they solve because they interfere with the normal operation of macOS.