OK you are not following the instructions.
sudo bash mount /dev/md126p5 /target cd /target mount -t proc proc proc/ mount -t sysfs sys sys/ mount -o bind /dev dev/ chroot . apt-get mdadm grub2
OK you are not following the instructions.
sudo bash
mount /dev/md126p5 /target
cd /target
mount -t proc proc proc/
mount -t sysfs sys sys/
mount -o bind /dev dev/
chroot .
apt-get mdadm grub2