DIAGNOSIS That's the low water alarm. The unit's self-evaporative system needs water in its internal tank to function properly, and E2 means it's dry or the float sensor is stuck.
FIX First, turn the unit off and unplug it. Locate the water tank (it's the removable plastic container at the bottom front). Pull it out, fill it with clean water to the MAX line, and slide it back in firmly until it clicks. Plug the unit back in and power it on. The E2 should clear after a few seconds.
IF THAT DIDN'T WORK If the code persists with a full tank, the float sensor inside the main unit housing might be jammed. You'll need to open the back panel (requires a screwdriver) and check the small white float in its vertical tube. Gently move it up and down to make sure it's not stuck on debris.
HEADS UP In very dry climates, you might see E2 more often because the air doesn't provide enough moisture. Running the unit in 'Cool' mode (not 'Dry') and keeping the tank full helps. Never run it with an empty tank for long, as it can damage the pump.