My iMac freezes after automatically installing 2021-006 Catalina update.

Security updates are generally not a problem, unless you have per-existing issues or have installed some third party AV junk-ware.

Try starting up in Safe Mode followed by a normal restart and test.

https://support.apple.com/guide/mac-help/start-up-your-mac-in-safe-mode-mh21245/mac

They may generally not be a problem, but this one is. I downloaded the security update and it froze on installation. My solution was to reinstall the Operating system, a total pain. I am hesitant to reinstall this update.

I assume you mean you booted into safe mode?

BTW, this is not a solved problem.