I installed zorin today after many years and this lack of option made my life difficult. I've searched a bit and I found that this isn't new and I wonder why this option isn't exist? If I remember well I used to bypass it by install grub on root partition in systems with legacy bios only. Now on UEFI this doesn't work and I ended up with grub to be installed on the current EFI partition and replaced the already installed and configured grub. I've managed to restore my previous grub, but why you don't add this option?
So, You have installed Zorin beneath an already installed other Linux System, yes? What System is it?
Yes, the system was already triple boot with windows, debian and archlinux. I installed zorin in place of debian.
And the GRUB was from Debian or Arch?
The installed grub was from arch. I had debian installed from arch with debootstrap via chroot, so I didn't install grub and I used the arch's grub to boot to debian.