Button.textContent was not set to string value :

   // ERROR => '[Delete]'
   // button.textContent = [Delete];
   button.textContent = '[Delete]';

With correction, code gives 100%.

;-)

function addItem() {
    // Select input field and read value
    const input = document.getElementById("newItemText");
    // console.log(input);
    // Create new <li> element and set its content to input value
    const liElement = document.createElement("li");

    liElement.textContent = input.value;

    // create delete button
    const button = document.createElement('a');
    button.href = '#';

    // ERROR => '[Delete]'
    button.textContent = '[Delete]';

    button.addEventListener('click', removeElement);
    liElement.appendChild(button);
    // Select <ul>  and append new <li> element
    document.getElementById("items").appendChild(liElement);
    // Nice-to-have: clear input field 
    input.value = "";

    function removeElement(ev) {
        const parent = ev.target.parentNode
        parent.remove();
    }
}