Multiplayer/Coop doesn't work because you need to create an online profile which is impossible because you need a valid CD Key. Activision's internal CD keys ran out and it seems like they can't be asked to restock them so you're basically getting a dud copy of the game whenever you buy it off steam.

If you already have this game on steam, the only way to play MP is to buy a sealed physical copy of the game and use thE CD key it came with to create a working profile.

DO NOT BUY THIS EVEN ON SALE!

fun

this game is funn