I'm not that clear as to what I am supposed to put in here

Tell us what’s happening:

Your code so far

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

Your browser information:

User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.0.2 Safari/605.1.15.

Challenge: Improve Compatibility with Browser Fallbacks

Link to the challenge:

Ahh, I see, what you have provided is correct. If you are not passing the test it probably has something to do with your browser. Try a different one.