I get an error "[Environment: Sandbox] when I try to purchase a subscription

A number of customers have reported this issue. It turns out that most if not all problems occur only on jailbroken iPhones.

According to this article there is a procedure to fix this:

Uninstall appsync
Remove Touring Mobilis Pro app
Reboot iPhone
Reinstall Touring Mobilis Pro
Reboot
Buy subscription via in-app purchase
Reinstall appsync
Now in-app purchases should work from now on