APFS not booting on MAC Mini 2014

I created a version of Catalina on a usb stick using my Air Book 2017 and it boots to it ok. But on my Mac Mini 2014 it see it as an option to boot from but when I restart the Mini it ignores the usb stick and boots from the internal drive. It this a problem with the Mac Mini and APFS drives?

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What version of macOS are you currently running on the Mac Mini?

Sierra