I think I know what method you are using. If it is the one I am thinking of, then if there are no results then null is returned. You are calling length method on null, which leads to your error.

You don't even need to filter the whole array, just the first 3 numbers.

Also wondering this, because I can't run his code on my IDLE (Python 3.6) since it gives me either an AttributeError or ValueError depending on None, num, or alphanumeric input.

I feel that there is only one true answer (at least in Python) since at least one of the test case has a ridiculously large n value.
So if anyone is stuck, your answer must be O(1). Luckily, I remembered that I did this problem in my algorithm course.

I was also stumped, but re-read instructions and it's probably related to:

Also, beware of plateaus !!! [1, 2, 2, 2, 1] has a peak while [1, 2, 2, 2, 3] does not.

So imagine you are working with an arr like [1, 2, 2, 2, 3, 3], where the second to last number (the first 3) is giving you a false positive.

Edit: and also imagine if you are working with double plateaus such as [1, 2, 2, 3, 3, 1]. In this case, the peak should be the first 3, but it's possible that the first 2 is giving you a false positive resulting in 2 peaks, when there should only be 1 in this case.

Hopefully final edit: I created a test scenario which your code might not pass according to your errors: [1,1,2,2,3,5,5,5,3,3,3,1,2,3]. There should only be 1 peak, but your code might give 2. Just passed this, so hopefully this helps.

Great example which emphasizes how concise ES6 can be compared to older ES versions, such as the top answer for this kata.