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unfortunately this is normally due to a catastrophic internal transmission failure such as a failed valve body, solenoid issue or simple failure of reverse gear
99% chances are the transmission will need to be opened for this repair.
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as I outlined, there is more than one thing that can cause this
if it is one of the bigger ones, you will be best served by having the gearbox fully overhauled or replaced
if by chance it is a simpler issue such as a solenoid or even low fluid level, then this will not require such a large fix
whether or not a large fix / overhaul / replacement is required cannot be said until the cause is conclusively identified