Diagnosis
That's the oven's main control board throwing a generic error. It's usually triggered by a sensor reading it doesn't like, most often from the temperature probe.
How to Fix It
Unplug the oven for a full 10 minutes. This does a hard reset on the main logic board. While it's unplugged, wipe down the metal temperature probe inside the oven (it's a little metal nub on the top rear wall) with a dry cloth to remove any grease or debris. Plug it back in and try a simple function like Toast.
If That Didn't Work
The error can also be caused by a faulty door switch. The oven thinks the door is open when it's not. Listen closely for a faint *click* when you close the door. If you don't hear it, the micro-switch behind the latch might be stuck or failed.
Heads Up
If the error keeps coming back after a reset, it's almost certainly a hardware fault with the probe or main board. The 'Err' code itself doesn't specify which, so official repair involves swapping parts to diagnose.