For some strange reason my right back door or my gas cap won't lock when I lock the car, I hope someone can help with this issue... thanks in advance!

 

If it’s the factory gas cap door, this is normal. The gas cap door does not lock like the vehicles doors.

 

No Honda Fit has automatic door lock for the gas door. In fact no Honda has a power button gas door anywhere in the history of the company. lol

You either push the lid. Or it works on a lever in the interior via cable.