Diagnosis
That's a sensor error. The unit's internal humidity sensor has either failed, come unplugged, or is giving a reading the control board doesn't trust.
How to Fix It
First, unplug the unit for a full 5 minutes to let the control board reset completely. Plug it back in. If E1 comes back immediately, the sensor itself is likely bad. On this model, the sensor is usually a small probe near the coils. You'd need to open the cabinet (unplugged!) to check its connection or replace it, which is a more involved repair.
If That Didn't Work
If the reset didn't clear it, and you're not comfortable opening it up, the control board might be the culprit. A persistent E1 after a hard reset often means a component replacement is needed.
Heads Up
Don't just ignore the E1 and try to force it to run. It uses that sensor to control the compressor cycle, and running it with a faulty sensor can lead to freezing up or not dehumidifying properly.