Я думаю, это должно быть следующим образом:
def input_Numbers():
number1 = int(input("Please enter a value for Number 1 : "))
number2 = int(input("Please enter a value for Number 2 : "))
return number1,number2
def calculate_Average(number1,number2):
average = (number1 + number2)/2
return average
def output_Average(average):
print(average)
number1, number2 = input_Numbers()
average = calculate_Average(number1,number2)
output_Average(average)