Door warning light?

Hello I have a 2011 Movano van 2.3 diesel 3500 FWD. when I get in my van and shut the door the computer is telling me that the door is still and open which is followed by the beeping noise, I would then have to open the door and close it to stop the beeping. the problem has gotten worse to the point where no matter how many times I open and shut the door the warning light stays on and when it comes to locking the door it just does a clicking noise but doesn't actually lock the doors. please can somebody help me as it is driving me nuts.