将列表作为参数传递给wolfram mathematica

I have a problem. I need to calculate the mean, but I don't understand why the code below doesn't work. I read the documention and this function should accept a list and return the mean value. What am I doing wrong? Why can't I pass a list to a function this way? How should I do it to get the correct result? Thank you in advance.I have a problem. I need to calculate the mean,