Diagnosis
That's the ambient temperature sensor error. The unit thinks the room temp is below 41°F (5°C), which is its minimum operating limit.
How to Fix It
Move the dehumidifier to a warmer room. The sensor is internal, so you need to warm the entire unit. Unplug it, let it sit in a heated space (above 60°F is safest) for a few hours, then plug it back in. The E1 should clear once it senses adequate temperature.
If That Didn't Work
The sensor or its connection might be faulty. Check for any obvious damage to the unit or the power cord. Try a different, known-good power outlet. If it persists in a warm room, a hard reset sometimes helps: unplug, wait 10 minutes, plug back in.
Heads Up
Don't try to run it in a cold space like an unheated basement in winter. The coils can frost over instantly, damaging the compressor. This unit is designed for continuous operation in spaces above 41°F.