getElementByClassName 不起作用

let elem = document.getElementsByClassName('card');
let warn = document.getElementsByClassName('copied');

for (let i = 0; i < elem.length; i++) elem[i].onclick = function() {
  warn.style.display = 'block';

  setTimeout(function() {
    warn.style.display = 'none';
  }, 3000);
}

.copied {
  display: none;
  position: absolute;
  bottom: 40px;
  right: 40px;

  padding: 10px 30px;
  border-radius: 8px;
  background-color: #8dffc0;
  font-size: 14px;
}

.cards {
  display: flex;
  width: 100%;
  align-items: flex-start;
  flex-wrap: wrap;
  justify-content: space-around;
}

.card {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  width: 250px;
  height: 150px;
  border: 1px solid #c4c4c4;
  margin-top: 5px;
  font-size: 28px;

  cursor: pointer;
}

<div class="copied">Скопировано</div>
<div class="cards">
  <div class="card" data-clipboard-text="¯\_(ツ)_/¯">
    ¯\_(ツ)_/¯
  </div>
  <div class="card" data-clipboard-text="☚(ﾟヮﾟ☚)">
    ☚(ﾟヮﾟ☚)
  </div>

  <div class="card" data-clipboard-text="٩(●ᴗ●)۶">
    ٩(●ᴗ●)۶
  </div>
</div>