Hi,

> The first thing to try would be to boot the installer media into rescue
> mode and try reinstalling grub. Or chroot the installed system and run
> grub-install and update-grub.

There is no installed grub in this case so there is nothing to
re-install. There is no installed system to chroot to as well.

> If this is a uefi system, you might just need to replace
> grub-efi-amd64-signed with grub-efi-amd64 while you're in chroot. Remember
> to 'apt update' first.

Ok but I'd need to install something to chroot to. So this is not the case here.

--
Regards,
Yevgeny