Unable to use my international Visa card on Apple Store to buy an iPhone

If you want to purchase from the US online store, you must use a US issued card with a US shipping and billing address. If you attempt to order with an international card or have a device shipped to a known freight forwarder, the order will be cancelled.

If you are physically in the US, you can purchase devices in a physical Apple Store using your international card.

This is a user-to-user forum. You are not speaking to Apple.

Payment, Financing, Refunds and VAT - Shopping Help - Apple

If your card was declined and you want to find out "why", you will need to take that up with Apple and/or with your credit/debit card bank.

If you tried to purchase an item on the US online Apple Store with an international card, the system flagged it as such and blocked the transaction. Or your bank did, probably suspecting fraudulent use.

You will need to contact your bank or Apple. We are users like yourself and cannot help you in this case.

Not sure if this would apply in your case, but Apple does not ship internationally, so if you were trying to buy a product and have it shipped to another country, Apple would cancel the order, no matter what bank issued the credit card.