The only Linux partition you have is sda6. sda2 is a windows partition, boot maybe?? and sda7 is a swap partition. It would be helpful if you posted the actual commands you use and the output as otherwise we have absolutely no idea what you did and what did or didn't work. You would create the mount point, mount the partition and install grub:
sudo mkdir /mnt/sda6
sudo mount -t ext4 /dev/sda6 /mnt/sda6
sudo grub-install --root-directory=/mnt/sda6 /dev/sda
Enter each line above separately and hit the Enter key after each.
This should work but not knowing what you did previously and the output you got, who knows?? You may have messed up your windows by mounting sda2 previously. What file does it say it cannot find. Posting actual error messages would be helpful.