it said “there may be a billing issue with your mobile account”. but i want to buy things with my Apple balance,
If you use mobile phone billing as an Apple ID payment method, you may need to add funds to you SIM card balance.
if you want to use your Apple ID balance, set your Apple ID balance as your default payment.
See -> Add a payment method to your Apple ID
Add a payment method to your Apple ID – Apple Support (UK)
- if you want to use your Apple Account balance, you need to set your Apple Account balance as your default payment.
See also -> If you need to change or update your Apple ID payment method
If you need to change or update your Apple ID payment method - Apple Support
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.