The Security Update 2021-006 Catalina is not installed after several attempts.

Appreciate that and since this appears to be Mission Critical setup Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Over-Kill ? - think not in this type of setup.

Download the Security Update from here,

https://support.apple.com/kb/DL2085?viewlocale=en_US&locale=en_US

Once downloaded double-click to install.

I should have come to this forum first. I have a 2019 Mac Pro and waited quite a while to do this update. It would not load ( I left the computer running overnight and was still frozen on the load bar 10 hours later. My computer would not start after that, and I had to do a system re-install from my time machine back up. So glad that I backed up before I TRIED to install the security update. What the heck Apple?????

Once the suggested method of Manually downloading the Security Update is done - Stop there. Do another TM Backup. Restart in Safe Mode and once in run the installer.

Find the Safe Mode is more reliable and eliminates all Third Party Software, extensions and fonts from playing up with the update.

Thanks so much for the reply. I am a bit gun shy on doing this. Took me all day yesterday to get everything back and running. I am a media composer and there were a lot of moving parts, plugin authorizations etc. that needed to be re-authorized, even after a successful Time Machine backup.

I actually do. Three SSD drives with time machine and an offsite NAS. That is how I got everything back to square one yesterday. The BUMMER is that for whatever reason, Time Machine will not point my Spitfire Audio Sound libraries to the correct added internal SSDs on my Mac Pro. I have to go through each library and reauthorize them. This, I believe is a Spitfire thing and not a Mac thing. It is a SLOW and BORING process, which took me over 2 hours to do yesterday. Other than my software update mishap, the Mac Pro has been rock solid! I went from an iMac pro with nine slave PCs down to a single machine. LOVE IT!