Possible reasons:
*Transmission fault:
* Breakage of reverse gears
* Wear or breakage of reverse friction discs
* Torque converter malfunction
* Electrical faults:
* Failure of the gear selector position sensor
*Short circuit or open circuit in transmission wiring
*Transmission control module malfunction
*Hydraulic faults:
*Low or contaminated transmission fluid
*Hydraulic valve malfunction
*Hydraulic pump failure
* Other reasons:
* Worn or damaged brakes
*Differential problems
*Physical obstruction in the transmission
Note:
It is highly recommended that you contact a qualified mechanic to accurately diagnose and correct the problem.