This is some old advice back from the last time Apple were having Apple Pay issues (which we were assured had been resolved months ago)
- Delete your Monzo card from Apple Pay
- Go to:
Settings
>iCloud
- at the bottom of the screen,Sign out
- Go to:
Settings
>Touch ID & Passcode
-Turn Passcode off
- Restart your iPhone.
- Sign back in to iCloud
- Turn Passcode back on
- Add your card to Apple Pay again
We were working directly with Apple to try and resolve these issues so this should work as long as you have the latest version of the Monzo app and iOS.