删除是在一个元素上,你有一个集合。
const allButtons = document.querySelectorAll('.button-56');
allButtons.forEach( btn => btn.remove() );
<button id="start" class="button-56" role="button">Start</button>
<button class="button-56" role="button">Quiz List</button>
<button class="button-56" role="button">Contact</button>