Puzzle #20 on Harry's world
You play piano on this puzzle and it's the same puzzle where you can get this achievement:
After you solve the puzzle, the piano plays a tune and you have to replicate that on the other side.
In front of piano press keys (white ones) in order from right to left: 7, 2, 4, 9
If you get it right but do not advance, it's likely you didn't press the keys fast enough.
This does not have to be first try.
Edit. If you downvote, please state why and I can improve this solution