First of all instead of setting string = s simply loop through s as it is. Also, instead of looping through both s and vowels just loop s and check if the letter is in vowels, if so add a white space to string if not add the letter to string. Here is the code:
def w_space (s):
vowels = "aeiouAEIOU"
string = ""
for a in s:
if a in vowels:
string += " "
else:
string += a
return string
solved How do I convert all the vowels within a string to white-spaces in python