Sounds like the NFC is not working on your iPhone. You’ll need to contact Apple Support and confirm your NFC is working properly.
If you have a heavy duty case on your iPhone you can try removing the case and attempting a transaction.
You’ll need to have Apple perform diagnostics on your iPhone. It can be done in person at an Apple Retail Store or at an Apple Authorized Service Provider. You can also contact Apple Support and request remote diagnostics.
If you’re in the US, please call 1-800-MY-APPLE, or outside the US, consult the support articles below. In Canada, call 1-800-263-3394.
Contact Apple for support and service - Apple Support
Genius Bar Reservation and Apple Support Options - Apple
Explore Service Locations
Official Apple Support