My EFI-partition is
/dev/sda2, but the following command does not work:
mount /dev/sda2 /boot/efi
It says it cannot find the mount point. I also tried
/mnt/boot/efi (doesn't work either) and
/mnt/ (works, but grub cannot find the EFI-directory when attempting the steps afterwards).
I skipped the installation of GRUB during the Debian installation because it failed to find my Windows installation and I suspected that it would break the EFI partition if it is not aware of a Windows installation.