I'm going to leave this for the more AO3-tech-savvy mod to check out inn the morning (as she may be asleep at this time of night). However, for now, I would just advise you to put that Peeta is a preferred/requested character in your optional details field somewhere. Since we don't match on characters, it won't affect your sign-up either way.

If the other mods can figure it out, we'll let you know so you can edit it. (I tested it on my own assignment and i'm baffled as well, because Peeta shows up in the drop down menu, but gives me the same error.)