Why can’t I purchase in game currency using apple account

Did you already try the troubleshooting mentioned in this related article?

If your payment method is declined in the App Store or iTunes Store

In case your payment information seems to be correct and complete, and you still can't purchase, you should follow the link in the error message and contact Apple Support. Only they can find the issue by looking into your personal data.

Subscriptions and Billing - Official Apple Support

Also, not all content can be bought, using Apple ID balance.