NFC not working in iPhone XR iOS 15

I get it's not working. When you buy a used phone however, you don't know its history or if the reason someone sold it was because certain things no longer work?

There's nothing anyone on this user to user only forum can do to help you further. Schedule an appointment as I suggested above.

You usually don't have to tap to pay. What you do, is once you've pulled up your wallet, selected the card and signed in, hover your iPhone over the area until you hear a ding. That let's you know the payment went through. It's not instant tap to pay and I never tap to pay.

I have never had to touch the NFC reader to get a payment to go through.

If it's not working, schedule an appointment at your Apple Store Genius Bar and have your phone looked at. iPhone XR is getting up there in years at this point. Your phone is used and may have been tampered with along the way.

Do you meant the side button is not working?

When you double tap the side power button what happens?