Hey JoanieH
the only thing I can think of would be secure boot, with secure boot enabled you can only boot from UEFI capable devices. So you would need to disable Secure boot, and enable Legacy support. This is how the defaults look.
Once you disable Secure Boot and Enable Legacy support - you'll reboot and the PC will ask you to confirm that YOU made the changes and you have to physiallcy type in a randomly generated 4 digit code... it will be displayed on the screen... even though it would be much more fun to not display it and make you guess what it was... ;)
Once you do that it should allow for non-UEFI aware devices (LINUX and other Legacy boot devices) to boot. I hope...
good luck
bill