Hi Wish555,
Often the message and situation you are describing -- "in-app purchase cannot be completed...contact iTunes store for assistance" -- is due to the payment or credit card information that Apple has on file for you either not working or needing to be updated.
Before stressing to much, I would check your iTunes account and verify that your payment/credit information is accurate and up-to-date and then re-try your in-app purchase.
To access iTunes payment account:
• Open iTunes on your PC/Mac
• Select "iTunes Store" located under "STORE" (usually found in the left-side window(s) selection area
• Make sure your in the "Home" area of the iTunes Store (Home button looks like a little house at the top of the iTunes app screen)
• On the right side of the Home screen there should be a list of links called "QUICK LINKS"
• Under this heading you should see the clickable "Account" link
• You may be asked for your iTunes Store/Apple login information and once you have entered that you should be taken to your main Account page where you can see your payment information at the top of the screen and other account options below.
If you're a visual learner, this may help navigate the iTunes Store and Account areas: http://note.io/19g2Dne
Best of luck!
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