look on the display when you crank the truck and try and drive it and tell me if the far right icon light with the forks/updwnarrows/slashmark is lit up? Also is the neutral indicator on the left side flashing?
If you see these indications then the seat switch circuit is not working.
The operator presense system is not activated and the truck does not see you on the seat with the seat belt buckled.
Check the wiring going to the seat, the connector is under the hood on the RH side near the hinge area, it goes across underneath the hood sound insulation and comes out through the hood and runs under the seat on the LH side, it connects to the seatbelt and also to a switch mounted under the seat cushion. There are a couple of screws accessible under the front side of the seat, remove those and you can remove the cushion and inspect the wiring and seat bracket assy.
I've repaired many of these because the wiring comes loose and as the seat moves up and down the wiring gets damaged. Also at the connector under the hood it can get damaged there also.