Hey, that happened to my OBS. Yes, it could be the same problem with your Impreza.
Take a look at the clutch pedal. About halfway up you'll find the plug thingy. I believe it's white. If it's broken or missing, your cruise won't engage because of the switch there. If you need a new one, it's only about $2 or $3.
Other possibilites are loose vacuum hose or bad fuse.