DIAGNOSIS That's the controller's catch-all error state. It usually means the auger motor or the fan motor is failing to initialize on startup, which is a pretty common hiccup with these.
FIX First, do a hard reset. Unplug the grill from the wall, then also disconnect the controller cable from the back of the unit itself. Wait a full minute. Plug the controller cable back in firmly, then plug the power cord back in. If the 'ERR' is gone, try starting it normally. If it persists, the issue is likely a failed motor component that needs replacement.
IF THAT DIDN'T WORK If the hard reset clears the code but it comes back when you try to ignite, the problem is often the combustion fan. Listen closely during startup—if you don't hear the fan whirring to life, that's your culprit. It's a known failure point and a replacement part you can install.
HEADS UP Don't just keep trying to start it over and over. If the fan or auger isn't working, you'll just flood the fire pot with pellets, creating a big, stubborn mess you'll have to vacuum out before you can try any fix.