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
