I have a Chevy 4 door Sedan

If the new key does not have a chip. It will not work. It must have a chip in it to work.
If it has a chip, then try to start the car and remain with the key on. Let it sit for up to 10 minutes or until the security light goes out. Repeat this process 3 times and the bcm will learn the pass code.

Roy

This uses a Passlock system so it has no chip in the key and merely changing the key will not require and programming. You can try the relearn process but I doubt that is your issue.

AUTO LEARN
Time required, approximately 10 minutes:

1. Insure that the battery is fully charged.
2. Use the scan tool in order to clear the DTCs.
3. Turn the ignition switch from the OFF position to the CRANK position attempting to start the vehicle. The vehicle will start and then stall.
4. Leave the ignition switch in the ON position while observing the security indicator.
5. When the security indicator turns off, which can take up to 10 minutes, turn the ignition switch off. Wait 10 seconds.
6. The BCM and the PCM will learn the new code on the next start attempt.