Well, if you look at the error message you see that the tester tried to call your function with the input [0,1] but got the response [6,10,14] instead of [0, 2]. Why it got [6,10,14] is not completely obvious. What [6, 10 , 14] is however is pretty obvious. That is the result from doubling the elements in the list 'n'.
What happens here is because you return 'n' instead of 'x' in your function. You then run it once before the tester gets it's turn, resulting in changing the contents of 'n' into [6,10,14]. The tester runs it's first test double_list([0,1]). It changes the object to [0,2] as it should, but instead of returning it you return 'n' once again.
(Short answer: return x instead of n)