Possible reasons:
Gear selector problems:
* Worn or damaged selector cable
* Incorrect selector adjustment
*Transmission control module (TCM) problem
Transmission problems:
* Worn or damaged friction discs or reverse steel plates
* Faulty reverse solenoid
*Low or contaminated transmission fluid (ATF)
Problems with the hydraulic system:
* ATF leak or low level
* Clogged hydraulic filter
* Faulty oil pump
Electronics problems:
* Faulty vehicle speed sensor
* Problems with wiring or connectors
Diagnostics and repair:
* Check the ATF level and condition: ATF should be red and without a burning smell. The level must be as specified in the instruction manual.
* Inspect the selector cable: Look for signs of wear, damage or rust.
* Scan for trouble codes: An OBD-II scanner can identify codes related to the transmission or TCM.
*Check that the gear selector is adjusted correctly: This may require special tools and knowledge.
* Replace reverse solenoid: If the solenoid is faulty, it may prevent reverse gear from engaging.
*Transmission Repair or Replacement: If the friction discs or steel plates are worn or damaged, the transmission may need to be repaired or replaced.
* Hydraulic system repair: Repair any leaks, replace a dirty filter and check the functionality of the oil pump.
Please note: Transmission diagnostics and repairs require special knowledge and experience. It is recommended that you contact a qualified mechanic for proper diagnosis and repair.