I am not able to add my Visa credit card as my payment method.

That does not necessarily mean that your card does not have money on it. Not all debit card are accepted for iTunes payment, it could be a name issue, it could be an address issue, the expiration date may not be correct, or something else. However, the only way to get it straightened out is to contact iTunes support. Use this link and find your location and use the contact information there. Choose your country or region - Official Apple Support

What is happening when you try to enter a card? Is it being declined? If so, see this, If your payment method is declined in the App Store or iTunes Store - Apple Support

no, it isn’t declining. Every time I try to add my card to my phone it says,”There is an issue with your payment method. Update your payment information to correct the problem and try again.” But I don’t know why...My card has money on it.