Hello Saku3210,
My apologies, I was never able to upload the Mac and Linux versions of the game to Steam, so the key will only work if you require a Windows version on the game. It is certainly something I can try to look at again in future, but not something I have the options for currently.
Unfortunately, I cannot edit the text on the main page of the game to inform people of this - that text is added by itch - though I have clarified this in the game's description.
Sorry for any inconvenience caused