Your function is return an empty array because your if conditional is never met. In the example given by this challenge, arr2 contains every element that arr1 has. So there will never be a point where arr2.indexOf(arr1[i] === -1). Even if arr1 contained elements that arr2 did not, your function would still miss any element in arr2 that arr1 did not contain.
You’ll need to find a way to see the unique elements in both arrays instead of just one of them.