I have fund on my Apple ID but still unable to purchase

cause u have to have a valid payment method,

a SIM card,credit or debit card or PayPal

check on settings-name-payment and shipping add a shipping adress and a payment method.The charge will be made to ur credit store till it will be 0 then the charge will happen on payment method

https://support.apple.com/en-us/HT201266

Do you have enough money to cover the cost plus any applicable taxes? What happens when you try to purchase something? Error message(s)?

If necessary, contact iTunes Support for assistance. See the Apple Support articles below for contact information.   

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