index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <link rel="stylesheet" href="style.css" />
  </head>
  <body>
    <div class="container">
      <p>Enter your text or URL</p>
      <input type="text" placeholder="text or URL" id="qrText" />

      <div id="imgBox">
        <img src="" id="qrImage" />
      </div>

      <button onclick="generateQR()">Generate QR Code</button>
    </div>

    <script>
      let imgBox = document.getElementById("imgBox");
      let qrImage = document.getElementById("qrImage");
      let qrText = document.getElementById("qrText");
      function generateQR() {

        if(qrText.value.length > 0){
          qrImage.src =
          "https://api.qrserver.com/v1/create-qr-code/?size=150x150&data=" +
          qrText.value;
          imgBox.classList.add("show-img");
        }
        else
        {
          qrText.classList.add('error');
          setTimeout(() => {
            qrText.classList.remove('error')
          }, 1000);
        }

      }
    </script>
  </body>
</html>