Unable to add fund to apple account

Funds are charged to the payment method you have added to your Apple Account. In other words, a credit or debit card that is attached to your account. You can change cards or order/priority of withdrawal by changing the order at this path,

iPhone > Settings > Apple Account (your name/photo at top of screen) > Payment & Shipping > Payment Methods

Once funds are added to your Apple Account, they are generally non-transferable and non-refundable.

Add money to your Apple Account balance - Apple Support

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

How apps, content, and subscriptions from Apple are billed - Apple Support

How are you trying to add funds, gift card, bank transfer or how exactly? What error message do you receive?