I've just tested this & I think everything is correct. Bare butts are definitely ok (in Day 15 anyway), as long as there is a face (lips & eyebrows) & no nipples.
Are you supposed to get it wrong multiple times? I tried the same thing again now and it said that I got one wrong and don't let it happen again. And then it continued to the next level.
I don't suppose you could tell me exactly which bit it was? What did Ingrid say exactly afterwards? It's possible I've got this wrong and need to fix it, but can't find the bit you mean