Our substitutions are not {user_id}, they are [user_id]. We do not use curly brackets. I've also no idea where you are trying to use this substitution. Substitutions in CB Content Module for example are always going to be from the viewing user and you have to use [cb:userdata field="user_id" userr="#displayed" /] if you want displayed user, but this only works on profiles and recommend using [cb:userdata field="user_id" userr="#displayedOrMe" /] so it switches depending on if a displayed user is available.