In your code, sum isn't a variable you can check, it's a callable funciton. You need your sum function to return the value of the two numbers. As roganjosh put, input() gives you a string, so you need to convert that to a integer to add them together.
a = input("enter number 1") b = input("enter number 2") # Instead of printing c, you're now returning c as the value from this function. def my_sum(a,b): c = int(a) + int(b) return c print(my_sum(a,b)) if my_sum(a,b) >= 100: print ("answer is more than 100") else: print("answer is less than 100")