Apple pay will not work with watch ultra

Ensure you follow the setup instructions in the Related Article: Make purchases using Apple Pay

Next ensure that you have not set up accessibility feature that may inhibit it.Recently I had turned on AssistiveTouch in Accessibility. I had turned it off and Apple Pay was not working from the Watch,  I turned AssistiveTouch back on then turned OFF the “Confirm With AssistiveTouch”, then turned off AssistiveTouch, and the ApplePay worked again, showing the “Hold Near Reader” when you double tap the side button.

If that is not the reason check your cards are OK

Reasons your payment card may be declined

1.            Account suspended for suspected fraud.

2.            Address in your iTunes account does not match exactly the billing address for the card

3.            Error in credit card number

4.            Insufficient balance or you still owe money on a subscription

5.            The card was not issued in the country of the iTunes store

6.            Wrong expiration date

7.            Wrong security (CVV) code

8.            You are connected through a VPN provider that makes iTunes think you live elsewhere - uninstall VPN

9.            You live  in India

About billing for Apple subscriptions and media products in India – Apple Support (UK)

10.        You live in Russian Federation or use Russian banks or cards

Here is additional information from Apple if your payment method is declined.

If your payment method is declined in the App Store or iTunes Store - Apple Support

After that you ought to reach out to Apple Support directly. You should be able to reach them here 

Click here > Contact - Official Apple Support

or phone numbers 

Click here > Contact Apple for support and service - Apple Support