2019 21.5” iMac won’t boot with new external SSD hard drive.

I just cloned my 2019 21.5” iMac 5400 RPM hard drive to a new OWC Mercury Elite Pro Mini using the free SuperDuper! The process went as expected. I set the start up drive to be the new external drive. Now it won’t boot up from the new drive. It gets about 7/8 of the way and then goes blank. I tried holding the shift key down to go into safe mode but it still won’t boot. How can I get the system up so I can set the internal HD up to be the start up drive and start over? While SuperDuper! was copying files Catalina updated to 10.15.6. I thought I stopped it from updating but apparently not.

Help!