sudo chroot /mnt
root@ubuntu:/# mount -a
root@ubuntu:/# sudo update-grub
Sourcing file `/etc/default/grub'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-5.0.0-27-generic
Found initrd image: /boot/initrd.img-5.0.0-27-generic
Found linux image: /boot/vmlinuz-5.0.0-23-generic
Found initrd image: /boot/initrd.img-5.0.0-23-generic
grub-probe: error: cannot find a GRUB drive for /dev/sda1.  Check your device.map.
Adding boot menu entry for EFI firmware configuration
done