i have a 100 dollars in my itunes. but when i make purchase in my game, it says decline. please fix it

See -> Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

• set your Apple ID balance as your default payment.

If you're still unable to update your payment method, Contact Apple Support.

Click here for more support and assistance.

Hey there,

what is the error message that’s you see exactly?

Do you see this error message “ Your Purchase Could Not Be Completed” ?

if so you have to contact Apple Support for help.