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Hello and welcome to JustAnswer!
I see that your question has been up for an extended time without a response and while although this vehicle is not in my area of greatest expertise I would like to try to help.
There are a few things that could be causing the problem. It could be wiring or the module that controls the locks, but the most common cause of locks not working on a single door would be a failed actuator, which is in the latch assembly. There is also an ajar switch in the latch, if the ajar switch isn't working then the car won't know the door is opened and the alarm wouldn't activate.
It's possible that the problem could be the latch itself, it could be unplugged or there could be a wiring issue. I'm not sure if it's something you're wanting to try and diagnose and repair yourself or not, the first step if wanting to diagnose it yourself will be to remove the door trim panel and use a test light or voltmeter to check for power and ground at the latch when pressing the lock and unlock button. If there is power and ground but it won't lock or unlock then the latch will need to be replaced.