Hey there,
AFAIR the message “error: variable prefix isn't set.” isn’t even the main problem - these are just the last messages one sees when the boot gets stuck in the very next step. On that IBM x3650 M2 mentioned at the bug, that message was printed several times. I think it was once per possible boot device, but I’m not sure.
Any chance that a firmware update is available for that Beelink Z83 II?
You could also try the new UCS 4.2, which was released a couple of hours ago.