Piano keys

Hi, i’m on step 10 on the piano keys app and i don’t see why it doesn’t change the letter but yes change the nextTwo display. any help?

nextOne.onclick = () => {
  nextTwo.hidden = false;
  nextOne.hidden = true;
  document.getELementById('letter-note-five').innerHTML = 'D';
  document.getELementById('letter-note-six').innerHTML = 'C';
}