I understand you want to create 3D plot of the errors vs x_true and y_true.
First of all, based on the code you have provided, the dimensions of x_pred and y_pred do not match those of x_true and y_true.
You can try evaluating x_pred and y_pred as below
x_pred = (Net_Out(1,:))';
y_pred = (Net_Out(2,:))';
For this kind of plot, using stem3 would provide the kind of plot you are expecting
stem3(x_true, y_true, err);
Documentation :
