Which edition of ZorinOS do you have e.g. Pro, Core, Lite etc? Can you edit your forum profile to indicate it there.
I don't think Timeshift is included in any edition of ZorinOS, only Backups (Déjà Dup). But I could be wong, as I only have Core not Pro.
Have a look at first post in this thread. It includes case for installing Timeshift.