maybe you had a typo? or they have corrected this in the meantime? I just tried this problem and it was error free.

def unique(values): """Finds all unique elements of a list.

        >>> unique([])
        []
        >>> unique([1, 2, 1])
        [1, 2]
        >>> unique([1, 2, 1, 3, 4, 2])
        [1, 2, 3, 4]
    """
    # your code here
    return list(set(values))

it also works without the coercion to list. return set(values) is fine.