DIAGNOSIS That's a continuous humidity sensor error. The unit's internal sensor that reads room humidity is either faulty, disconnected, or giving a nonsensical reading.
FIX The first fix is a full system reset. 1. Unplug the dehumidifier from the wall. 2. Press and hold the Power button on the control panel for 30 seconds while it's unplugged. 3. Still unplugged, remove the water tank. 4. Leave everything unplugged and disassembled for at least 10 minutes. This drains residual power and resets the main board. Then plug it back in, insert the tank, and see if the error clears.
IF THAT DIDN'T WORK The sensor might be physically obstructed. Unplug the unit and remove the filter. Look near the top of the cavity for a small plastic nub or vent—that's the sensor. Gently vacuum any dust off it. Do not touch it with your fingers or spray anything on it.
HEADS UP If the error code returns immediately after the reset, the sensor or main control board is likely defective. The unit needs professional service. You can't fix a failed sensor yourself on this model.