background-color: red: in the wrong block.
The root is where you set the variable. The variable is called in .red-box but if a browser doesn’t recognize the variable you need to provide an alternative.
If you reset the code you’ll notice that in the .red-box block there’s a empty space before the
background: var(--red-color); for you to put in the answer.
Oh, and btw, it’s just background…it told you that. NOT background-color
While this doesn’t entirely cover what you reported in 404 Error Reported about there not being a Hint page it at least answers the problem you’re seeing.
Read the instructions and guides carefully. HTML and CSS do what you tell them. It can be hard and frustrating when you don’t see the error you’ve made at first glance so take a step back. Get a sammich and then try looking again with fresh eyes.