Late 2012 27-inch iMac failing to update Catalina

A friend’s 2012 27" iMac hung in the middle of updating Catalina and got caught in a loop trying to complete the update. Restarting just hung again.
Finally, I booted from an external drive with an SSD and tried to reinstall Catalina on the internal SSD. The computer reported that “macOS could not be installed on your computer” and suggested starting up while holding “D” which failed with Error: 0x8000000000000003, Cannot load ‘EFI/Drivers/TestSupport.efi’ Status: 0x00000003.
I was able to use Carbon Copy Cloner to copy the MacSSD Data to a different external drive. Disk First Aid gave the internal SSD (at each level) a green checkmark, but it still refused to install Catalina. So since the data was copied and we had other backups from before the problems, I erased the internal SSD and tried to install Catalina which still failed. Thanks in advance for any suggestions.

Might be worth trying to reset the PRAM with the appropriate keyboard boot up action - command-option-p-r while machine boots up.

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Thanks. He did that, but it won’t hurt to do it again.