Not sure exactly which Golden Tee you have, but if it uses a battery, it being dead might cause the loss of credits, especially if other settings are lost as well when the game is powered off.
The low voltage message could just be low +5 voltage from your power supply, so if it has a +5 adjustment you can meter the +5 at the board connector and adjust it to +5 or a little over (like +5.1).
IIUC, some boards have had issues with power draw at the connector and have bypass connectors to supply power to the board. If your board has them, make sure they are securely connected.