Originally posted by burn:

are you playing on a multiplayer server or just single player?

^ This, most commonly. Devcomands are not - and have never been - accessible on multiplayer (usable only by the host in client-hosted games and blocked completely for maps hosted by the dedicated server tool) without the use of a mod.

Alternately, there are some outdated mods that interfere with the use of the console, making devcommands unavailable.