SRFGRID not accepting u values

Namaste All,
I am trying to make a surface by generating random no. of points and moving them randomly in z direction, but the srfgrid component is showing error when in u value.Kindly guide.
srfgrid (12.9 KB)

You are better off using a real grid of points, not just some populated random points.

srfgrid (17.1 KB)