What to do if there is a problem installing OMLx
You can attach the calamares-installation-log.txt to your post here or in a bug report.
On some pc’s you have to enable UEFI or not in BIOS, ofc I have no clue about Apple stuff. But the Grub2 Menu is not supposed to have UEFI or Legacy boot choice that is dynamically configured somehow. Beyond that this would be question for developers.