Learn Responsive Web Design by Building a Piano - Step 6

I don’t know what’s wrong with my code, can’t get past this, please help me

/* file: index.html */
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <title>Piano</title>
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
</head>
<body>
  <div id="piano">
    <div class="keys">
      <div class="key"></div>
      <div class="key" class="black--key"></div>
      <div class="key" class="black--key"></div>
      <div class="key"></div>
      <div class="key" class="black--key"></div>
      <div class="key" class="black--key"></div>
      <div class="key" class="black--key"></div>
    </div>
  </div>
</body>
</html>
/* file: styles.css */

  **Your browser information:**

User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.0.0 Safari/537.36

Challenge: Learn Responsive Web Design by Building a Piano - Step 6

Link to the challenge:

You will need to give your class attribute 2 class values. 1 of them already exists