Diagnosis
That's the "Door Switch" error. The fridge thinks one of the doors (likely the refrigerator side) has been open for an extremely long time, usually over 48 hours. It's often a faulty door switch sensor.
How to Fix It
First, check if the door is actually sealing. Make sure nothing's blocking it and wipe the gasket clean. Then, you can try to reset the error. Unplug the fridge from the wall for 2 full minutes. Plug it back in. The error should clear. If it comes back immediately, the door switch itself is probably bad and needs replacing.
If That Didn't Work
If the error persists after a reset, the issue is likely the door switch, a small plastic plunger mechanism inside the door frame. You can test it by opening the door and listening for a faint click near the top hinge when you manually press the plunger in and out. No click means it's dead. It's a common part you can order and replace yourself.
Heads Up
Don't ignore this error. The fridge might stop trying to cool properly if it thinks the door is always open, which can lead to food spoilage.