If your vehicle is showing single door fails to lock or unlock with the power button, here are the most likely causes and the parts you'll need to fix each one.
The small electric motor or internal plastic gears inside the specific door have worn out or stripped, preventing it from physically moving the lock mechanism.
The wires running between the car body and the door flex every time the door is opened. Over time, the specific wire powering the lock actuator can fatigue and snap.
The metal rod connecting the actuator to the door latch was left disconnected, bent, or improperly clipped during a recent window regulator or door handle repair.
Dirt, rust, or dried-out factory grease has caused the physical latch mechanism to jam, making it too stiff for the electric actuator motor to move.
The main computer that controls body electronics has suffered a rare internal failure on the specific circuit or driver that sends power to that single door's lock.