What is the exact error you receive?
Thanks, ***** ***** you trying to make a purchase in the Mac App Store on your Mac or the App Store on an Apple iOS device?
First, try signing out of your account in Settings > iTunes & App Store. Tap the ID listed, then sign out.
Once signed out, go ahead and sign back in. Next try the purchase once more.
Ok, next make sure your date, time, and time zone are correct in Settings > General > Date & Time.
I'm sorry, I can't offer telephone support at this time.
However, if the iPad shows the correct time on screen then it should be fine.
Are you able to download other apps and/or updates?
From a computer, test out that it is working properly by signing into the Apple ID management site:
https://appleid.apple.com/ (Click "Manage Your Apple ID")
Are you able to sign in?
Ok, next are you able to make the purchase via iTunes on your computer?
Ok and are you able to see the money balance in iTunes?
You should not have to disable any other device as the credit should be usable from anywhere if you are signing in under the same ID.
Can you make the purchase from the app store under iTunes on your computer?
Open the iTunes app on your computer.
You should see three dots near the play button (just below), click it and choose Apps.
Next choose "App Store"
Then locate the app you wish to purchase and try it.
That's no problem.
Feel free to reply back at any time for additional help in tackling this issue.