Cannot pass challenge even though my answer is correct

The problem is that you are removing items from a list while looping over that same list. Doing that will lead to items being moved around in the list while it is being looped over, which will result in some item being skipped by the loop.

The code mainly fails in cases where a vowel appears repeatedly, for example if you pass the string "aaaa" to your function it will return "aa" which is clearly not correct.

This can be fixed by using the string itself for the loop source instead of the list, like this:

def disemvowel(word):
    vowels = ["a","e","i","o","u"]
    vowels = vowels + list("".join(vowels).upper())
    letters = list(word)
    for letter in word:
        if letter in vowels:
            letters.remove(letter)

    word = "".join(letters)
    return word

Also just as a tip you can perform this task without having both lowercase and uppercase versions in the vowel list by calling lower on the letter as you compare it to the vowels list. Like this:

def disemvowel(word):
    vowels = ["a","e","i","o","u"]
    letters = list(word)
    for letter in word:
        if letter.lower() in vowels:
            letters.remove(letter)

    word = "".join(letters)
    return word

Which at least to me feels a bit cleaner than adding more letters to the vowels list.