I am missing a button as well, and also don't see one on Groupees. I don't really remember how I bought this cause I have bought so many bundles.
Do you see the game at all in your groupees purchase page? It's just "Spell Casting". If you don't have it, then you don't have it. Maybe you got the itch.io key in gift or trade? In which case you'd want to contact the person who you got the gift link from.