Once upon a time, the Qubes recommendations were to set your BIOS to boot legacy-style. That may have changed but wasn’t the last time that I looked. It may be a little harder to determine how it is phrased in your BIOS but try to look for legacy boot as an option or some variation. UEFI always will have pressure behind it to be used but for quite a while now the part of the Qubes automated testing that has been a soft fail has always been the UEFI portion.