reverse not working

If reverse is the only gear affected then I suspect we have an issue in the reverse clutch piston or seal. Basically inside the transmission there is a piston that applies reverse only and if this is cut or failed in other ways then you will not have any reverse but all the other gears will operate.

Below you will find the process on how this works.

I also attached the process on how to check this and possible causes. Please run through this material and let me know if you have questions. Thanks