If you have UA miles, then you book with UA and UA.com. Seats are indeed available on partner airlines, just not every flight, every day especially at peak travel times. And since SFO is a hub and a large market, award travel from SFO is not as good as from other markets. You'll have to be flexible and try other west coast gateways like SEA, YVR or DEN. Then try to improve as departure date approaches and last minute award seats are released.