I want to add money to my account

See the following Apple articles:

Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

Add money to your Apple Account balance

Add money to your Apple Account balance - Apple Support

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.