Great game. Took me a while to figure out how to use the piano!
a lot of people are struggling with this, i may have to update the description with a hint since i won't have time to change anything in the game before the lock happens. in essence though, pressing X will play a note if there is a note key pressed. you will know you did it right if a piano note plays.
EDIT: nvm i just pushed an update that hopefully clears up some confusion :)