Actually, you can
Are you trying to do it as a multi-stop itinerary AUS-IAD-GRU-GIG? If so, that's probably why it won't price out, because United can't fly you from GRU to GIG as a stand-alone ticket. However, if you look for AUS-IAD-GIG, it'll price it out just fine for you. UA3773 AUS-IAD, connecting to UA861 IAD-GIG (with a stop at GIG).