Apple Pay not working

This might help;

Annoyingly, even if you've already set up Apple Pay on your iOS device, you'll need to add your cards again to your Apple Watch. Luckily you can use the iPhone's camera to capture the data from the front of the card.

1. Open the Apple Watch app on your iPhone. Make sure you're in the My Watch tab.
2. Scroll down and tap Wallet & Apple Pay.
3. Tap 'Add Credit or Debit Card', then follow the entry fields on the next screen.
4. Confirm that the details are correct.
5. Tap Next.
6. Wait while your bank or card issuer verifies your information.
7. Once your card is verified you can use Apple Pay on your Watch.

You may have a card already set up for your Apple ID, in which case you may just be able to enter your security code.