If you have made sure that the charging cable is unplugged and there is no other error in the on-board computer, the brake light switch is probably dead.
The brake light switch is responsible for determining whether you are pressing the brake pedal or not. When it is pressed, the brake light comes on. Since you can only engage in to gear when you press the brake pedal, you can no longer engage in to gear if this switch is defective. There are now two solutions to get the smart ready to drive again: