For people who cannot pass the piano room, I provide you a save file right after the piano puzzle.
Go check local files and find a folder called Savefile after piano, open it, you will see a save file. Copy paste it into www > save. It will replace save file No.10.
Load that file and you can continue the game. And you can use this file to achieve both normal ending and true ending.
I really don't know why that thing is not working for some people. I apologize for the inconvenience.