Diagnosis
'Flow Error' means the system detected an imbalance between the incoming water and the outgoing wastewater. Even if the drain looks okay, the sensor thinks it's not.
How to Fix It
The most common fix is to clear the flow restrictor in the drain line. Disconnect the grey drain line tubing from the saddle valve at your sink drain. There's a small plastic restrictor piece inside the tubing or at the connection. Soak it in vinegar to dissolve any scale, then rinse it thoroughly and reconnect.
If That Didn't Work
The flow sensor itself on the main board might be faulty or have a wire connection come loose. You'd need to open the electronics cover (with the power off) and check the sensor's wiring.
Heads Up
Make sure your drain line has a proper 'air gap' or is looped high before descending to the drain. A direct siphon can cause flow errors too.