The SCU will ask for the supervisor code if:
1. An operator manually enters a vehicle FieldID but Manual Authorisation has been disabled on the MCU/SCU.
You will have to go into the scu config screen (Button C for Cal) and enable manual entry.
2. The SCU has lost comms to the MCU.
3. K Factors are missing.
4. SCU.crp file is corrupt (zero bytes). A good test for this is if Alphanumeric keypad entry is on but you only see numbers - you have a corrupt SCU.crp file.
Quit the MCU app, remove the file and restart the app - NOTE that you may need to re-enter K Factors.
First course of action is to VNC into the MCU and check that the MCU can communicate with the SCU and that the K Factors are present.