Eh guys before starting to come with stupid answers I have same problem running steam in admin and the game itself running as admin from the steam library
And also getting steam must be running ,still it is better then yesterday waiting for a treyarch update to go online that still hasent happend
Also for Skidrow games, you need to follow these instructions to resolve this issue. https://firstandgeek.com/steam-must-be-running-to-play-this-game/