Do you see a "Your purchase could not be completed" message?
A likely reason for seeing this message is Apple may require you have a credit or debit card present on your account too, even if it won't be used to make the purchase.
Use the document Add a payment method to your Apple ID - Apple Support to review your payment methods such as credit or debit cards. Problem methods will usually have a notice in red lettering below them. The article: If your payment method is declined in the App Store or iTunes Store - Apple Support has some information about declined payment methods.
If that does not work, you may find the fastest route to resolution would be to accept the invitation to contact Apple for assistance with this.
To contact iTunes Support, do this:
1. Click on this link --> Choose your country or region - Official Apple Support
2. Select your country
3. Select: Billing & Subscriptions
4. Select: Subscriptions & Purchases
5. Select: Update payment method
6. Select a chat session or a phone call.