This is not a Test User account.
Please create a new account in the Sandbox environment. [Environment: Sandbox]
Then the purchase will fail.
Game Center also has Sandbox problem. Once you log in using your Game Center account, you will see a red "Sandbox" label on top of the Me icon in Game Center. Once this red "Sanbox" label appears, all your Game Center friends are gone. For example, you can no longer visit your friends in the popular Dragonvale game.
After searching the Internet and tried many different ways to fix, finally I found a way which completely fix the problem. Following is the the detail steps:
1. After the jailbreak, a app called Cyndia should already installed on your iPad or iPhone.
2. Open Cyndia, select "Sources" tab, click on Edit button at top-right corner and then click the Add button at top-left corner. Add the following resource:
3. Select "Search" tab, and search "AppSync". Select the corresponding version and install. For example, if you iPad is iOS5.1.1, choose AppSync for iOS5.x+.
After install is done, you can select Restart Springboard.
4. Search "Installous" and select Installous 5 and install. After install is done, you can select Restart Springboard.
6. Try the games and application which have the Sandbox problem. The annoying Sanbox problem should been fixed.
If the problem still there, you can uninstall and re-install these games or applications and the problem should be solved.
Enjoy your Sandbox free iPad or iPhone!
As a bonus, the Installous app you installed at step 4 has lots of free apps and games.