This question already has an answer here:
If you look at the seating chart for that plane, you'll notice that those windows match the location of toilets and galleys.
Note that the exact position of toilets and galleys may change during the lifetime of an aircraft, so it's easier to have all the windows installed by design, and just obstruct some that are not used. Even if it were just obstructed from within, I suppose it's better if in an emergency, if emergency personnel had to look inside the plane (to check for people still trapped inside), they would be able to identify quickly those windows that do not need looking through.
It means that there is some sort of internal structuer (like a wall or a cooking unit) behind that window on that particular configuration of the aircraft. Manufacturers are required to certify the planes with windows in place in order to show that they don't suffer from metal fatigue after repeated flights. So moving a window (or covering it up with metal) is a lot harder than just putting a little plastic cover on the inside of the window. Hence, when a particular configuration places a wall or a cooking unit there, they just cover it up.