In the test “Basic CSS: Attach a Fallback value to a CSS Variable”, this is the task:
Add a fallback value of black to the background property of penguin-top and penguin-bottom classes.
In the HTML page (where we make the actual changes), the word penguin is misspelled twice.
Under classes penguin-top and penguin-bottom, it reads: var (- -pengiun-top), var(- -pengiun-bottom), respectively.
If you change the var to the correct spelling (pengUIn vs pengIUn), you get an error. So, the solution seems to include this misspelling.
(luckily, I misspelled one and not the other so the error kind of discovered itself).