I connected a 1K remote potentiometer, with the VFD 10v to one leg of the pot VI to the Wiper Center leg and other leg to ACM. I was able to control the speed using the potentiometer with PD001 = 1 and PD002 = 1.
However, once I hook up the VI and ACM to the phoenix board, I am unable to control the spindle speed. It seems the VFD is not the problem. The problem might be the CNC controller or grbl settings.