Hi Sumplus,
welcome to the Apple (user-to-user) Community.
Your Apple ID is the account you use to access Apple services, like the App Store, Apple Music, iCloud, iMessage, FaceTime, and more.
How to: Add a payment method to your Apple ID - Apple Support
Re: "I can't add funds to my Apple ID"
Wondering if you have tried this method:
How to: Add money to your Apple Account balance - Apple Support
and
What you can buy with your Apple Gift Card or Apple Account balance - Apple Support
says:
"Some purchases might require that you have a credit card on file,
even if you use your Apple Account balance to make the purchase".
All the best :-)