External USB Start-up Disk won’t boot - Circle with a line through it icon?

I’m still running Sierra on my 2012 MacMini. I’ve largely stuck to that because I’m running an older version of InDesign and it seems to work well. I’m at a point though where I’m thinking of updating to Mojave but figured I’d do an install of Sierra onto an external usb drive for the rare occasions that I need to work with InDesign.

I used SuperDuper to make a copy of my internal ssd. It shows up as a bootable drive in System preferences but when I attempt to restart using that usb drive I get a circle with line (“Prohibited” icon according to the Apple page on start-up icons) at the end of the process. Any thoughts on the fix?

Running the latest version of Sierra. It’s a powered hard drive formatted as Mac OS extended, journaled, GUID partition. USB connected directly to Mac, no hub.

I have seen this where you can’t EXTernal boot
“newer” than what your machine shipped with

(Think 2012 was Mountain Lion?? So you
could EXTernal boot ML but anything newer
won’t go) However that doesn’t prevent you
from upgrading to currently supported OS
for your hardware

Also, if you have replaced your mini’s drive, with
an SSD, (or any non-apple drive) EXTernal boot
is hit or miss.

Did you open a support ticket at Shirt Pocket? They are usually pretty quick and helpful with answering customer questions?