why does it keep saying my card is declined when there is plenty of money on the card to pay for the extra storage upgrade

According to Apple:

How to complete your purchase if a payment failed or was declined:

1. On your iPhone or iPad, open the Settings app.
2. Tap your name.
3. Tap Payment & Shipping.
4. Add a different payment method and remove the old one. 
5. Try to complete your purchase.

If you still can't make a purchase:

• You might have an unpaid order. Learn how to pay an unpaid order.
• Contact your financial institution. 
  • You might need to ask your financial institution to allow international purchases on your debit or credit card.
  • If you want to know why a debit card, credit card, or other payment method was declined, speak with your financial institution. Apple doesn't have information about why the payment method was declined.
• In some countries and regions, your payment method might be declined because of changes to local law. Learn about billing and purchases in Europe or what to do if your card is declined in India.

Need more help?

• Call 1-800-APLCARE. 📞
• International Support. 🌐
• Contact Apple Support. 📝
• Live chat using Messages. 💬