Hi there, seppe272.
I see that you added some funds to your Apple ID balance. Afterwards, you tried to make an in-app purchase for a game that you play, but you keep getting a message that the purchase couldn't be completed and to contact iTunes Support for help. I'm glad to help with this.
View your Apple ID balance -- Double-check your Apple ID balance to confirm the amount that's there.
If your payment method is declined in the App Store or iTunes Store -- If the amount was sufficient, you may or may not have seen these messages, but the steps in this article can help.
Payment methods that you can use with your Apple ID -- This article provides the information regarding the payment methods you can use. Be sure to review country- or region-specific information under "Payment methods by country or region".
Apple - Support - Product Selection — If you're still getting a message about an incomplete purchase at this point, please contact iTunes Support for help. After selecting this link, choose Billing & Subscriptions > Subscriptions & Purchases > Unable to purchase. Then select the method you'd like to speak to them with.
Take care.