Thanks for the question, and great troubleshooting so far. Let’s try the following in order:
1) Force close the App Store and iTunes applications
2) Sign out of your account in Settings > iTunes & App Store
3) Restart your device
4) Check to make sure your date and time are correct
And lastly, sign back into your account via Settings > iTunes & App Store. Once signed in, tap your account name then tap “View Apple ID” (you may be prompted for your password once more). This will take you to your Account Settings page, where you can view if you have any open orders that need to be settled before you can make new purchases.
Thanks for the follow up! If your issue persists after following the above resources, you may find more information by using another one of Apple's support channels - https://getsupport.apple.com/GetproductgroupList.action
- Matt M.