I have a Maax spa and I emptied the water yesterday and refilled it. However upon powering it up, it goes into priming mode but then after it displays "dr" which I found out it means inadequate water detected in heater. I even empited and refilled the spa again but no luck. How can I go about fixing this?

Well, the first thing you need to figure out- is there indeed adequate water flow over the heater? If so, then maybe just a bad sensor.

If there isn't adequate flow, then maybe a dirty filter, pump that's stubborn to get a prime, or bad pump, etc, etc.