How to add credit to my account

You can transfer funds following the directions in this support article:

Add money to your Apple Account balance - Apple Support

You can also redeem Apple Gift card funds to the account.

How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

Lastly, you can add a debit and/or credit card to your Apple ID account.

Add a payment method to your Apple ID - Apple Support

Are you trying to add funds to Apple Cash or Apple ID Account?