at 4:30, my clock does NOT do countdown what does it mean?

this is my index.js:

let timer = 10 
function decreaseTimer(){
   // infinite loop = requestAnimationFrame(animate)
  if (timer > 0) {
    setTimeout(decreaseTimer, 1000)
    timer--
    document.querySelector('#timer').innerHTML = timer // innerHTML = cái số 10 á 
  }

  if (player.health === enemy.health){
    console.log('tie')
  }
}

and this is my index.html:

<!-- timer -->
      <div id="timer"
      style="
        background-color: red; 
        width: 100px;
        height: 100px;
        flex-shrink: 0;
        display: flex;  
        align-items: center; 
        justify-content: center;">
      10
      </div>

I keep getting this error for cancel timeout! index.js:134 Uncaught ReferenceError: cancelTimeout is not defined

at determineWinner (index.js:134:3)

at animate (index.js:212:5)