Clean-out plug in p-trap won't screw back on
This sounds like the plug in a floor drain. If so, sometimes the threads get mucked up. Often you can just clean them with some Limeaway or another decalcification product. If that's not working go to your local hardware store or HD and buy an expandable rubber drain plug. These plugs are about 1 or so inches in diameter and have a wing nut on one side and a metal washer on the other. You pop it in the drain plug hole and as you tighten the wingnut the rubber between the wingnut and the washer compresses and expands. Works like a charm.