https://wiki.mageia.org/en/Converting_a ... ed_systems
System is easy to revert to legacy BIOS mode if the BIOS will not accept Mageia as an installed UEFI system.
Change back to legacy in the BIOS, rerun the install the same way, just delete the /boot/EFI partition you just created.
Some notes at the end regarding a Mga8 legacy to UEFI conversion run.