Fix
This is most likely a connector wiring harness problem and is easily fixed by you like I fixed mine. It happened to me last week on the passenger side. The door first failed to close and may have had an obstacle in the way near the closed position. Then for the next few days, would simply beep and give the "Sliding Door Power Unavailable" message each time I tried to operate it. Key Fob, door outside button, pillar inside button, overhead console button, and door handle operation, all would do the same thing. I went to this forum, read this thread and suspected the connector. However, at the time I had no certainty of what caused it. I am experienced and familiar with this type of connector failure. It is caused by a combination of poor contact pressure, pin contact surface corrosion or contamination, and thermal cycling and relaxation of the contact pressure. It is aggravated by temperature extremes, age, poor pin material selection or connector design, humidity, and electrical heating. As the connection degrades, the contact heating increases and the problem snowballs. It happens most to power pin connections. This car obviously has current sensing for the door motor that reverses the door when an obstacle gets in the way and loads the door motor (high current limit exceeded for a certain length of time). Current sensing (too low as well as too high) is also used all over the car to detect component failures. One such failure occurs when there is a problem with the door power delivery, hence the message and disabling of the door electrical operation. This is to protect the components as well as alert you to a problem.
This type of failure is hard to diagnose because it is sometimes intermittent. But sometimes when it occurs, it does not go away and is easier to find like in my case.
Troubleshooting is difficult because disconnecting or even disturbing the connector can appear to fix it. Some may feel that this somehow reset things or it just remains unexplained (until it happens again).
A little explanation:
The connector that I suspect caused my problem is the one mentioned by others that is clipped to the plastic plate in the bottom of the door track. As has been pointed out, the red slide on the top of that connector is just a lock. it is not electrical. Operating it may have disturbed the connector enough to get it to work again. However, in my case, no amount of wiggling of wires, disturbance of the connected connector, or flipping this lock made any difference. So here is what I did that fixed it:
1. Grab the plastic plate that the connector is clipped to and pull straight up on it. This releases the plate from the two body studs so that you can pull it, the connector and its wires just out of the track area so you can manipulate it easily.
2. Slide the red lock forward (toward the front of the car).
3. Guide the forward wire bundle out of the retaining tabs of the plate so you will be able to pull the forward connector half free from the other half that is clipped to the plate.
4. Depress the connector lock while pulling the connector apart.
5. Apply bulb (dielectric) grease to the pins to keep them corrosion free and making good contact.
6. Push the connector back together all the way and then apart several times to help clear the contact surfaces.
7. Put it back together the way you took it apart, insuring the plate is clipped onto the studs as before and the wiring is in the restraint and neat in the bottom of the track.
8. Operate the door manually, then try it with a button.
I have a question for anyone else who is having this problem with one of the sliding doors. Before performing a fix, while the problem is active, can you try operating that sliding door's window? Of course, with the driver door window disable function NOT disabled. Since I do not have a wiring diagram, nor do I know where the current trip occurs, it might be instructive to FCA to know if this power disable occurs for all of that door's function or just for the door transit motor.