DIAGNOSIS The oven's internal computer has detected a fault, usually with a temperature sensor or the main control board. "Err" is a generic catch-all code.
FIX The first and most effective reset is a full power cycle. 1) Unplug the oven from the wall outlet. 2) Wait for a full 60 seconds—this lets any residual power drain from the capacitors. 3) While it's unplugged, press and hold the power/start button for 5 seconds (this can help discharge static). 4) Plug it back in. The display should light up normally. Try selecting a function like Toast or Bake. If "Err" is gone, you're likely good to go.
IF THAT DIDN'T WORK If "Err" returns immediately or after a few minutes of operation, the fault is persistent. The most likely culprit is a failed oven temperature sensor (the probe inside the cavity). This requires a service technician to diagnose and replace.
HEADS UP If you're getting "Err" specifically when using the Air Fry function, double-check that you're using the correct mesh basket and that it's seated fully and correctly on the drip tray. A misaligned basket can sometimes trigger a sensor fault.