The partition table format in use on your disks normally required you to create a separate partition for boot loader code. This partition should be marked for use as an @EFI boot partition@ and should be at least 35 MB in size. Note that this is not the same as a partition mounted on /boot

If you do not back to the partitionning menu an correct this error, boot loader installation may fail later, altought it may still be possiblile to install boot loaer to a partition.