Hi,
do you get any concret error messages when grub install fails? You might be able to get some messages/hints from the installation logs (and/or provide them here).
The following post describes how to grab the logs while in installer-mode: An attempt to configure apt failed
Manual installation of grub might be possible at the command line. You will have to “chroot” into the “/target” directory (in not already unmounted) and run “grub-install” with the necessary options. Sorry but I can’t go into much detail here because that highly depends on the system and the status of the installation. I would therefore focus on analysing the problem of the “normal” installation.
Regards,
Janis Meybohm