There a several solutions to this problem and all involve removing the seat and rear cover of the drivetrain. The brake and lever arm of the brake is located on the end of the black motor. Moving the brake lever up and down places the motor in freewheel or locks the motor/brake. There is a connecting rod from the brake lever to a ball that can be pulled up or down where it exits the back cover.
The connecting rod and brake lever may be bent making it difficult to work. Gently bend it back into alignment if you can. Be careful not to damage the brake and brake lever. The rod is easily reshaped. There is also a chance that the mechanism is binding and rusted or loose. You should be able to work the rod easily. See if you can move the lever right at the brake.
The more likely culprit is an overheated seized brake with the only solution replacing the brake. I have also seen the hexagon shaped nut which is secured by 2 set screws onto the motor shaft inside the brake housing come off. The set screws get loose and the nut gets lodged in the brake. Both of these solutions requires removing the brake with some allen wrenches after noting its position on the motor shaft. Hope this helps.