Read about clear. When you feel you are done with your previous code, then you can clear all your workspace variables and then run the fresh code. It looks there might be a variable conflict earlier when there was error.
My guess is that earlier in that code you created variables named sum and/or mean. Attempts to call sum and/or mean while variables named sum and/or mean exist generally will not work, they will be interpreted as attempts to index into the variable.
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