DIAGNOSIS That's a general controller error, often triggered by a brief power hiccup or a probe reading it didn't like. It's the grill's way of saying it got confused and needs a hard reset.
FIX First, turn the power switch on the front of the grill to OFF. Then, unplug the power cord from the outlet. Wait a full 60 seconds—this lets the controller's internal memory clear completely. Plug it back in, turn the power switch on, and it should boot up normally. If the 'ERR' is gone, you're good to go.
IF THAT DIDN'T WORK If the 'ERR' comes back immediately, the problem might be with the RTD (temperature probe). Unplug the grill again. Locate the probe inside the cook chamber, on the left side. Check that its wire connector is firmly seated into the socket on the back wall. A loose connection here can cause constant errors.
HEADS UP If the reset works but the error keeps returning randomly, it's usually the RTD probe failing. They're a common wear item. You'll need to replace it, but it's a simple unplug-old, plug-in-new part.