Hi Ali,
Great question! So the name of the expression editor input variables need to correspond with the variables used in the expression editor itself. On a side note, a variable can't have a "space" in them. Try to replace "Variable x" to "Variable_x". See attached image.
In your case, you just have to change the inputs of the Expression editor to "x","y" and "z", because those are the variables you used in the expression editor itself.
Thanks and good luck!
Arie
