The Oracle VM Server software image is preinstalled on the server in the Legacy BIOS Boot Mode. Therefore, to used the preinstalled image, you must boot the server in the Legacy BIOS Boot Mode (the default). If you boot the server in the UEFI BIOS Boot Mode, the server will not boot the Oracle VM preinstalled image and it cannot be used. Oracle VM does not support UEFI BIOS. If you want to switch to UEFI BIOS Boot Mode then you must install an operating system that supports UEFI BIOS.