I can't understand why I can't.

[Avante]

<button onclick="abbr();">Нажми меня для запуска</button>
<p id="example"></p>
<script>
  function abbr() {
    let a = 0;
    let timerId = setTimeout(Seconds, 1000, a);
  }
function Seconds(a) {

let t = document.getElementById("example");

t.innerHTML = a;

a += 1;

if (a > 4) {

window.close();

}

return a;

}

</script>

[elysha]

function abbr() {
  let a = 0;
  let timerId = setTimeout(Seconds, 1000, a);
}
function Seconds(a) {
  let t = document.getElementById("example");
  t.innerHTML = a;
  a += 1;
  if(a > 4) { window.close(); }
  return a;   
}

<button onclick="abbr();">Нажми меня для запуска</button>
<p id="example"></p>

What's wrong?

By goodness, your violin should be declared to the element in which it is used.

It is therefore necessary to cause/describe the required functions after the entire page is downloaded (onLoad)