Monterey upgrade failure

Is all your software 64 bit?

Is your data backed up?

Do you have the minimum of 45 GB free?

https://krypted.com/mac-os-x/free-space-required-for-modern-macos-upgrades/

If not, step back, and determine what needs to be on your system, and what is compatible first and make sure all your data is backed up in case the upgrade fails.

Check then the health of your system. Make sure with http://www.binaryfruit.com/ DriveDX you have no pending storage issues. Check with Etrecheck, you don't have any older memory resident programs that can be problematic:

Etrecheck - the handy reporting tool for … - Apple Community

Mojave is the last Mac OS to support 32 bit applications. My Catalina upgrade FAQ gives guidance for upgrading past Mojave:

Catalina - an upgrade guide - Apple Community

Make sure Filevault is turned off as this can delay upgrades.