Apple Pay is a mobile payment service that allows users to make contactless payments with their compatible Apple devices. If you use this service and now you are thinking why isn't my Apple Pay working then you should read below:
Your card issuer doesn't support Apple Pay
Not all banks and credit card companies support Apple Pay. You can check if your card issuer supports Apple Pay by visiting their website or contacting them directly.
You don't have the latest version of iOS, watchOS, or macOS
Apple Pay requires a minimum version of iOS, watchOS, or macOS to work. You can update your device to the latest version by going to Settings > General > Software Update.
Your card isn't set up correctly in Wallet
Make sure you've entered your card information correctly in Wallet. You can also try adding your card again.