Red Triangle, 2009 Prius

My 09(gen II) Prius has only 98k miles on it. Bought it from a dealership Thursday. I drove to work a few days and to my buddies apt last night just fine. But when I went to leave his apt, starts up just fine, reverses just fine, but as soon as I leave it in N or put it in Drive, that Red Triangle pops up /and the screen says “the vehicle must be in P to charge the battery.” I checked the battery consumption and there was at least 3/4 battery in it. I’m having it towed in case it’s serious.
Can anyone tell me if this is something to be worried about?