Why can't I buy Apple products on the official website
I use the balance to make a payment, and it keeps showing that I can't pay
iPhone 15
Question marked as
Are you trying to buy Apple hardware, that includes Apple Care plus?
In that case you can not use your Apple ID account balance to buy the hardware and the extended warranty plan.
What you can buy with your Apple Gift Card or Apple Account balance - Apple Support
When I bought my Apple Watch using Apple ID balance, I had to add the extended Apple Care warranty plan later, using my credit card.
Also, paying with Apple ID balance has to be supported for your region:
Payment methods that you can use with your Apple ID - Apple Support