StuDded issue, StdDev, unknown

When I try and use StdDev like in the example from the docs, I get an error thatthat stdDev is u known,/ does not exist…
What am I doing wrong

stdDev({Root}, “Weight”)

Where are you trying to use it? This is an expression function that can only be used in a property binding, not in any scripts.

ahh I was trying to use it on a button click. That must be it. That was driving me crazy thanks