This game is no longer for sale on Steam, but that should not affect the activation of this game. You just can't buy it any more. That said:

Once upon a time, a game key was a sacred thing: you bought it, you owned it. Not any more, alas. Keys get revoked these days for no reasons in the world.

This key is an old one. I keep track of these things as best I can, and my records say I haven't used it and I haven't given it away, so it should be as valid as the day I bought it. If it isn't, please let me know and I'll check to make sure there wasn't any mistake on my end. If I can fix the issue, I certainly will.