Why does P0420 catalytic code pop when the OBD11 checks the converter? 06 Pontiac Grand Prix 3.8 ?

A P0420 code means the Bank 1 converter is not working at it's minimum required rate. Replace the Bank 1 converter. It's the converter nearest the firewall.

It's not a bad 02 sensor. That code only sets when the 02 sensors are working and the converter is bad.

P0420 Possible Causes: 

Faulty Three-way Catalyst Converter Bank 1Exhaust TubeIntake Air LeaksFaulty Oxygen (O2) SensorFaulty Fuel injector(s)Leaking Fuel Injector (s)Faulty Spark plugsImproper Ignition TimingFaulty Engine Control Module (ECM)Dirty Air Filter

The cat converter has failed the efficiency test. In most cases, the problem is the converter itself. See this post on P0420

https://ricksfreeautorepairadvice.com/p0420-is-it-...

Check for an exhaust leak.  Check what the rear O2 sensor is doing when the engine is running and at operating temperature with a scanner.  You might need a new cat converter.