My code not working

def main():
    scores = input("Enter five test scores: ")
    scores = [int(x) for x in scores.split(",")]
    showLetters(scores)
    calc_average(scores)



def showLetters(grades):
    for x in grades:
        scores = [int(x)]
        if x >= 90 and x <= 100:
            grades = "A"
        elif x >= 80 and x <= 89:
            grades = "B"
        elif x >= 70 and x <= 79:
            grades = "C"
        elif x >= 60 and x <= 69:
            grades = "D"
        elif x < 60:
            grades = "F"
        print("{:.1f} is {}".format(x, grades))

def calc_average(grades):
    total = 0
    for el in grades:
        scores = [int(el)]
        total += el
        average = total / 5
        letter = showLetters(average)
        print("The average is: {:.1f} which is {}".format(average, letter))


main()

line 12, in showLetters
    for x in grades:
TypeError: 'float' object is not iterable