Why can't I set my Pulse conductivity units to ppm?

Ppm, or parts per million, makes sense as a measurement in solutions when you are controlling exactly what is present. For example, you will have RO, tap or bore water (typically a conductivity of less than 0.1 EC) to which you will add known nutrients and nothing else.

In soil however, when you are measuring the conductivity you will also be getting influences from everything around the plant, including what the plant itself has expelled, bacteria around the plant and other organic matter. As such you cannot guarantee that all the EC you are seeing is related only to the nutrients you have added where the ppm is known.