It seems like "Error code 31" is a specific error associated with older versions of SmartLocks. This error is not associated with the newer SmartLocks Pro.
If you are getting Error code 31, please follow the steps below:
Take the disc out
Reset the disc
Put it back
Make sure it is placed/connected correctly
Reset the Smart Lock (press and hold the button in the back of the lock)
Close the app
Turn off the Bluetooth
Turn the Bluetooth on, open the app and try again