Diagnosis
The E01 error is a sensor fault. The oven's internal temperature sensor is either disconnected or has failed.
How to Fix It
Unplug the oven and let it cool completely. Remove the crumb tray, drip tray, and all racks. Look at the back wall inside the cooking chamber. You'll see the temperature sensor probe near the top right heating element. Check if the wire is securely plugged into the main board. If it is, the sensor itself is likely bad.
If That Didn't Work
If the connection is good, the main control board is the other possibility. You'd need a multimeter to test the sensor's resistance. A faulty board would require replacement.
Heads Up
Do not attempt to run the oven with this error. It cannot regulate temperature safely. The sensor assembly is a specific part, not a generic thermostat.