I just got a sterling shower kit, I installed base & drain. The drain was made of plastic or pvc and it came with a rubber donut inside to slip in the 2" PVC, I said to myself this looks like something that will leak, Guess what? It did. Anyone have any ideas??

Have you checked to see if the "O" ring didn't bunch up when you installed the drain? If it did then reposition the "O" ring and lube the pipe. Good luck, Tom

Change to the one that you glue your 2 inch into