Possible reasons:
1. Gear selector lever operation error:
* The switch position is incorrect (it may not be fully in the ‘D’ or ‘R’ position).
* Worn or damaged linkage between the lever and transmission.
2. Malfunction of solenoids:
* The reverse gear solenoid is stuck in the activated state.
* Open or short circuit in the electrical circuits associated with the solenoids.
3. Problems with clutches:
* Worn or damaged friction discs that cannot reliably transfer torque in the desired direction.
* Lack of hydraulic pressure required to engage the clutches.
4. Problems with the torque converter:
*A damaged or worn torque converter may not transmit torque properly.
*Transmission fluid flow problems that affect torque converter performance.
5. Transmission ECU malfunction:
* The electronic transmission control unit, which controls gear shifting, may malfunction and send incorrect commands.
What to do:
* Check that the gear selector lever is positioned correctly.
*Consult a qualified mechanic for thorough diagnosis and repair. Diagnostics may include checking fault codes, mechanical checks and hydraulic tests.