You have used it correctly (almost). Remember we said there would be no while loop? I would choose a different variable name, though.
for letter in text:
You don’t need to convert this to a list since all we’re doing is iterating over it.
if letter not in 'aeiouAEIOU':
You are appending correctly but the only flaw is the use of the
str() function in your return. It will turn a list into a string, just not what we expect…
str([1,2,3,4,5]) => '[1,2,3,4,5]'
join method to convert a list to a string.
When you get this working, see if your can perform the entire operation using only strings and no lists. Again, no built-ins.