Sorry to hear you're having trouble purchasing Wifibooth.
I can help you fix this!
Here are the steps to take:
1) first, confirm that restrictions are turned off by going to Settings App -> Content & Privacy Restrictions -> make sure all restrictions are turned OFF.
2) next, go to Settings App -> iTunes & App Store, touch the Apple ID at the top, and then hit "Sign Out". Then
power off your iPad all the way (swipe to power off), power back on again, go back to Settings App -> iTunes & App Store, and sign back in with your Apple ID again. This step really does usually fix the problem!
Try purchasing Wifibooth again now.
If it still doesn't work, then the next step is to call the payment provider of your Apple ID, which is usually a credit card company. Call them, and ask them if they see an attempted charge or not. If yes, then ask them why it isn't going through. If not, then you need to get in touch with Apple Support at this URL here:
https://support.apple.com/HT203005
Did that work?