Help in pino keys project? did not work at all

My code is not working at all and I don’t know why ?

// The keys and notes variables store the piano keys

const keys = ['c-key', 'd-key', 'e-key', 'f-key', 'g-key', 'a-key', 'b-key', 'high-c-key', 'c-sharp-key', 'd-sharp-key', 'f-sharp-key', 'g-sharp-key', 'a-sharp-key'];

const notes = [];

keys.forEach(function(key){

  notes.push(document.getElementById(key));

})

// Write named functions that change the color of the keys below

function keyPlay(e){

  e.target.style.backgroundColor = "blue";

}

function keyReturn(e){

  e.target.style.backgroundColor = '';

}

// Write a named function with event handler properties

function assign(note){

  note.onevent.mousedown = keyPlay(); 

  note.onevent.mouseup = keyReturn(); 

}

// Write a loop that runs the array elements through the function

notes.forEach(assign)

// These variables store the buttons that progress the user through the lyrics

let nextOne = document.getElementById('first-next-line');

let nextTwo = document.getElementById('second-next-line');

let nextThree = document.getElementById('third-next-line');

let startOver = document.getElementById('fourth-next-line');

// This variable stores the '-END' lyric element

let lastLyric = document.getElementById('column-optional');

// These statements are "hiding" all the progress buttons, but the first one

nextTwo.hidden = true;

nextThree.hidden = true;

startOver.hidden= true;

// Write anonymous event handler property and function for the first progress button

// Write anonymous event handler property and function for the second progress button

// Write anonymous event handler property and function for the third progress button

// This is the event handler property and function for the startOver button

startOver.onclick = function() {

  nextOne.hidden = false;

  startOver.hidden = true;

   document.getElementById('word-one').innerHTML = 'HAP-';

  document.getElementById('letter-note-one').innerHTML = 'G';

  document.getElementById('word-two').innerHTML = 'PY';

  document.getElementById('letter-note-two').innerHTML = 'G';

  document.getElementById('word-three').innerHTML = 'BIRTH-';

  document.getElementById('letter-note-three').innerHTML = 'A';

  document.getElementById('word-four').innerHTML = 'DAY';

  document.getElementById('letter-note-four').innerHTML = 'G';

  document.getElementById('word-five').innerHTML = 'TO';

  document.getElementById('letter-note-five').innerHTML = 'C';

  document.getElementById('word-six').innerHTML = 'YOU!';

  document.getElementById('letter-note-six').innerHTML = 'B';

}