This challenge was updated very recently (last 12 hours), so you need to make sure you are using the correct function definition. There should only be a single parameter (n) defined for the function. I suggest clicking on Reset All Code and start again.
The initial challenge code should look like below:
//Only change code below this line
function countdown(n){
return;
}
console.log(countdown(5)); // [5, 4, 3, 2, 1]