If your card is not working on your iPhone, it may be due to the following reasons:
1. Your Visa card might not be supported by Apple Pay, preventing you from using it with the app.
2. Your card may work for other payments but not with Apple Pay.
3. You may need to update your payment information with your bank or with Apple.
4. Sometimes, your card may be temporarily locked, causing it to be declined.
5. Another possibility is that your card or transaction account has insufficient funds or a limit that is preventing the transaction.
6. Incorrectly entering your card PIN at the terminal can also result in a declined transaction.
If you're experiencing any issues, it's a good idea to contact your bank and inquire about the problem. They can provide further assistance and help resolve the issue with your card on your iPhone.