Sometimes when trying to make a purchase on the App store, users are presented with a message that says :
This is not a Test User Account [Environment Sandbox]
This error has nothing to do with the App and is a fault with your device settings.
You need to go to Settings / App Store and sign out of the App Store. Then you need to soft reset your phone and then you need to go to Settings / App Store and sign back in. This should fix the issue.
If you still have a problem after this, you may need to take your device to Apple.