I can't solve it

Tell us what’s happening:

Your code so far


<style>
  :root {
    --red-color: red;
  }
  
  }
  .red-box {
    
    background: var(--red-box, red);
    height: 200px;
    width:200px;
  }
  
</style>
<div class="red-box"></div>

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0.

Link to the challenge:
https://learn.freecodecamp.org/responsive-web-design/basic-css/improve-compatibility-with-browser-fallbacks

It is fairly common for campers to not understand this challenge at first. Bottom line is that not all browsers will understand css variables so be prepared for that