Hi,

changing the cube in the model drawing (modification by handles) means, you are also changing a parameter, but not by typing the new value into an input field, but by moving a mouse inside the viewport. But you are changing a parameter. "Building element has no memeber 'Length'" means, that the handle is defined so, that it is linked to a parameter "Length", but there is no such parameter defined in the PYP-

Best,
Bart