Now *I'm* confused! It was "Witness" that I was trying to add, and AO3 kept telling me it couldn't save the story because it couldn't find the community. And when I looked at the Beholder page, I didn't see "Witness"; it must have taken a few minutes to show up. Hmmm. Well, it's up there, so I guess all's well. Sorry for the bother!
(For some reason, the auto-fill wouldn't give me any options at all; just kept timing out.)