I had the same issue with Astarion when he was downed in a fight. He made his death saving throws so he was just lying there at 0 HP. I think a healing spell would have done the trick but none of my characters had one.
I solved it by attacking him (an option when you right-click on him, or presumably also if you ctrl-click on him). That killed him and then I was able to use the revivify scroll.
