Abstract:
The reason why you are getting this error message is because you are trying to call a method on an int type of a variable. This would work if would have called len() function on a list type of a variable. Let's examin the two cases:
Fail:
num = 10
print(len(num))
The above will produce an error similar to yours due to calling len() function on an int type of a variable;
Success:
data = [0, 4, 8, 9, 12]
print(len(data))
The above will work since you are calling a function on a list type of a variable;