rough idle

So, I have a 2015 2.0t. We replaced the timing chain and the clutch. Once we got it back together, it runs fine and accelerates good. but at idle, it starts running rough up to the point where the engine bucks and even dies. We have put new plugs in it, checked the coil packs, and checked all of the sensor plug ins. We are throwing a P0420 code.

We are trying to figure out if the catalytic converter is bad and causing the fuel/air mixture to be adjusted improperly, or if its something else that is causing the P0420 code.