Hi,
it looks like sda consists of a single filesystem, i.e. you did something like
mkfs.ext4 /dev/sda instead of formatting the disk with fdisk /dev/sda, creating
one or more partitions an then creating a filesystem on one partition:
mkfs.ext4 /dev/sda1.
However, this was maybe intended from you. In this case you need to
grub-install --force --boot-directory=/mnt/boot /dev/sda. As far as I know,
this kind of setup might not be bootable, because there may be no space at the
first 512 bytes of the disk. (Maybe I'm wrong, if ext4 leaves the first 512
bytes free.)
If you are doing the first solution, use bios and not uefi, there are 2 ways to
format a disk. Gpt and msdos/mbr. If you choose gpt, you'll need a second 1mb
partition for grub.
HTH.
Martin
