Can you see it now?
The only reason why you didn't see the button I can think about is that you'd got the game after this post was created.
Oh, my bad, it's said you got it in January.
It's weird it doesn't show up for you, cuz I can see how other players activate their keys.
You can text me on discord or find the purchase email from itch. I think the claim button will be there.
That's what Itch says about the keys, and there're enough keys stored on the platform: 