Code means it is not recognising the transponder in the key.
Can sometimes just be a one-off and taking the key out then putting it back in can solve it, we have similar sometimes on DAF’s and Merc’s where the key isn’t recognised.
The dealer should have suggested this course of action first. The code simply means the security code has not been received to allow the vehicle to start.
Expect an expensive bill from Merc, either a New ECU os some description or new transponder ring reader and or new keys as well.