I think Yann has you chrooted and running the dpkg. I post this when just running it from inside an working install to update grub. You need arrow keys & spacebar. Do not choose partitions and never ever choose a NTFS partition. In fact we have filed bug reports for years to get them to stop installing the PBR - partition boot sector of NTFS partitions as that damages Windows.
#to get grub2 to remember where to reinstall on updates:
sudo dpkg-reconfigure grub-pc
#Enter thru first pages,spacebar to choose/unchoose drive, enter to accept, do not choose partitions