Startup Security Utility has no options. Have tried several workarounds, but nothing works.
Have created bootable external SSD with macOS Big Sur for backup.
But that disk is not listed when I try to use it for startup disk.
Cloned the internal SSD using Carbon Copy Cloner. It has boot sector.
Am guessing that external SSD is not recognized by Startup Security Utility.
iMac Line (2012 and Later)