Please try the following steps:
1. Go to your iPad Settings app, and Select the "iTunes and App Store" option on the left.
2. Tap your Apple-ID to sign out of the App Store.
3. Now reboot your iPad as described here:
4. After it re-starts, go back in to Settings, and sign in to the App Store again.
5. Now go back to the app, and try your purchase again.
Your purchases should now work correctly.