How to enable the NFC on my iPhone 6S Plus

Double click the side button to activate Apple Pay and display your default credit or debit card.

Make purchases using Apple Pay - Apple Support

If you need instructions on adding cards to your Apple Wallet, please follow the step by step instructions in the Apple Support article I shared below.

Set up Apple Pay - Apple Support

What are you trying to use NFC for? Your phone released in 2015, uses NFC primarily for Apple Pay. To my recollection, there is no switch to turn it on or off.