the Librem-Mini machine comes with SeaBIOS + CoreBOOT (or PureBOOT/HEADS) not EfiBIOS that most proprietary motherboards from AMI do.
you can make a new GPT partition table under an unformated 512 MiB ‘bios-grub’ labeled partition and a second 512 MiB fat32 /bios partition on the Librem-Mini and it will install PureOS just fine.