During rescue mode, there can be problems with grub-installation if we are not able to ‘chroot’ in the correct OS environment.
so, if the rescue mode of a Linux OS allows you to run grub from the command line, then installing grub can be done like this

grub-install /dev/sda –root-directory=/mnt/boot

for this, MBR is on /dev/sda and the boot directory with the grub files is located/mounted at /mnt/boot