My key also didn't work, so I had just set it aside for years. It's not a problem, though, since you made the game free. (Thanks!)
In case it helps prevent similar problems in the future, my key was https://arquoia.itch.io/roa/download/grps-74rHTMHZPvYA3ZaAxYbKbfTTYP-Vcm4WjoXEYA...