It won’t let me make a purchase

If you supplied a credit or debit card, check its details. If they're correct, or you're using a different payment method, click here and ask the iTunes Store staff for assistance.

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Go to www.apple.com/support/itunes/ww/ as instructed. Select your region. Proceed from there.

Get help with an item purchased from the App Store, iTunes Store, or iBooks Store - Apple Support