The Most Wanted Letter.Need help!!!

def checkio(text):
    for letter in text.lower() : 
        if letter in "abcdefghijklmnopqrstuvwxyz":
            lst = [(letter,text.count(letter)) for letter in text]
            lst.sort(key=lambda x:(-x[1],x[0]))
        else : continue
    return lst[0][0]




        what's wrong ?