When attempting to purchase or restore items from the Shop, the error "This is not a Test User account. Please create a new account in the Sandbox environment" is displayed.
This error is likely being caused by the jailbreak or software modification on your device.
To get the in app purchases working again, try the following:
1. Uninstall AppSync from Cydia
2. Uninstall TowerMadness
3. Reboot your device
4. Re-download TowerMadness from the App Store
You should then be able to buy or restore items in the Shop.
If your device is not jailbroken, you may also try simply deleting and re-downloading the game from the App Store. Note that reinstalling the game will reset your progress.