When you run grub-install by default it assumes the EFI system is mounted as /boot/efi
It depends on your distribution where EFI system is mounted and on some distributions it isn't mounted after boot.
First check if /boot/efi is mounted with
mount | grep /boot/efi If that doesn't work first try the following to see if it is mounted elsewhere.
mount | grep /dev/sda6 If neither of those work do:
mount /dev/sda4 /mnt Now run:
grub-install --efi-directory=[efi dir] grub-mkconfig -o /boot/grub/grub.cfg where [efi dir] is either /boot/efi or /mnt