When you did
mean = mean(X)
that will work the first time, calculating the mean and storing the result in the variable named "mean".
But the second time you ask to execute it, mean(X) does not refer to the function mean, it refers to the variable "mean" that you assigned the calculation to.
Solution: never use a variable name which is the same as the name of a function you might want to call, because as soon as you do the assignment, the name stops referring to the function and starts referring to the variable.
