I think the reason why you can't tame it with crops/berries on mobile is that in one dungeon(ceremony of the ethereal stallion) you can kill it(it's the boss) and resurrect it for some 60 Amber. (I tried) The resurrection works, but it would be too easy if you could just tame it with crops. 😭But understandable. So, if you've gotten griffin kibble, but no unicorn, farm (or buy) some amber, go to the dungeon. And ressurect it.