Building photo gallery

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Photo Gallery</title>
    <link rel="stylesheet" href="./styles.css">
  </head>
  <body>
    <div class="header">
      <h1>CSS FLEXBOX PHOTO GALLERY</h1>
    </div>
    <div id="gallery"></div>
    <gallery>
      <img></img>
      <img></img>
      <img></img>
      <img></img>
      <img></img>
      <img></img>
      <img></img>
      <img></img>
      <img></img>
      <img></img>
    </gallery>
        </body>
</html>

pls i dont understand this challenge and it cannot execute