Diagnosis
That's the grill's generic error state, usually triggered when the hot rod igniter fails to light pellets in the allotted time or a temperature probe reads way out of bounds.
How to Fix It
First, power the grill completely off at the control panel. Open the hopper lid and the main chamber. Vacuum out any excess ash or pellet dust from the fire pot. Make sure your pellets are dry and flowing freely—clumpy, damp pellets won't ignite. Close everything up, power it back on, and try a startup cycle. The 'ERR' should clear if the igniter can get a clean burn going.
If That Didn't Work
The hot rod igniter itself might be dead. It's a consumable part. You can check it (when cool!) by looking into the fire pot during a startup attempt; you should see it glowing red within a few minutes. If it's not glowing, that's your culprit and it needs replacement.
Heads Up
Constantly getting 'ERR' on startup, even with fresh pellets and a clean pot, often points straight to a failing igniter. They do wear out after a year or two of regular use.