Please help. I don't know what I am doing wrong

Tell us what’s happening:
Describe your issue in detail here.

  **Your code so far**
/* file: index.html */
<!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">
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/1.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/2.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/3.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/4.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/5.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/6.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/7.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/8.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/9.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/10.jpg"/>
  </div>
</body>
</html>
/* file: styles.css */
* {
box-sizing: border-box;
}

body {
margin: 0;
font-family: Arial;
background: #EBE7E7;
}

.header {
text-align: center;
padding: 32px;
background: #E0DDDD;
}

#gallery {
display: flex;
flex-direction: row;
flex-wrap: wrap;
justify-content: center;
align-items: center;
padding: 0 4px;
}

#gallery img {
width: 25%;
height: 300px;
object-fit: cover;
margin-top: 8px;
padding: 0 4px;
border-radius: 10px;
}

@media (max-width: 800px;} {#gallery{width: 50%;}}
  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/102.0.0.0 Safari/537.36

Challenge: Step 20

Link to the challenge:

Take a look at the way you wrote the media query.

how am I meant to write it please?

It’s best if you look it up yourself for learning purposes, just Google media query.

Here is an example of how a media query can be written.

 @media screen and (min-width: 480px) {
  body {
    background-color: lightgreen;
  }
}

You’ve got the right idea but are using the wrong brackets in some places. Also it is better to lay the query out like the above to save confusion about where opening and closing brackets are.