Why my SIM card won’t work? In my iPhone 6s Plus version 12.0.1(16A404)

Why not? What happens when you insert the sim card into the iPhone?

Do you get an error? Or other onscreen message?

See here: If you get an alert about the SIM card in your iPhone - Apple Support

Maybe because it is locked to original carrier.

If so, you have to ask that carrier to officially unlock iPhone for you. Not Apple or anyone.

You should contact your carrier service if your SIM card doesn’t sork