As Shane said, all ranks are giving a set amount of LWCs to use to lock their chests, doors etc. As a builder you are given 15 locks (you can check with /climits) and as you progress through the ranks you are given more LWCs. You may also pay $15 USD to buy the "Umlimited LWCs" donation which as per the name gives you an unlimited amount of locks.
The above being said, LWC commands only working in the Rising, Legacy, and Aether worlds. If you're in the mining world or nether, you can't use them. Use /getpos to find out what world you're in.
