First, raise the wheels off the ground and block the frame to the ground so the truck doesnt take off. Stay well away from rotating drive wheels as it will break bones.
For electric shift models, first check the directional switches in the steering column. Must be switching correctly and not both directions engaged at once. Continuity, voltage, adjustment. Use a multimeter not a test light. Then check the shift solenoid(s). Pulser coils with the screw-on black coil you can check with a small screwdriver, the end of the valve stem should be magnetic if the coil is receiving power and is energized. check for power and continuity of the coil, if okay, possible valve problem (check main pressure on the trans control valve head to confirm, should be at least 120PSI... not sure of the exact pressure.) The other style of electric two-way coil may have a manual valve shift button on either side, press it and see if trans engages.
Also, check your inching adjustment. observe the action of the inching spool/bellcrank in response to the movement of the pedal. unhook and move it to its furthest at-rest position and see if the trans picks up a gear.
also, some trucks block engagement of a direction if the parking brake is engaged, so check operation of the handbrake switch.

this is all general advice for any trans not shifting into gear, for specific info someone else will have to chime in. my memory of these trucks is not sharp enough to recall what style you have which varies with the specific model number after the model code you posted (TFN etc.)