Easy.
Seen that a lot.
If the truck operates then when plugging it sometimes shows that replace the F/R sensor. Part # 140747.
If it shows it all the time then go into the service menu and go to A2.4.1 this will show your forward switch move the control handle from natural to forward the number shown should change from a 0 to a 1 move the handle several times it should change every time 0 in the neutral position and 1 with the forward command.
Then the same thing for the reverse. A2.4.2 should be the same response 0 and 1.
Not changing to 0 and 1 replace the same sensor as above.
If it is showing the right numbers then calibrate the traction pot. C1.1 if calibration fails then replace pot.