Make sure iOS on your iPhone is running the most recent version. Go to Settings > General > Software Update to verify this.
Restart your iPhone before attempting to complete the transaction. If you restart, do you still get the error?
The Apple Servers may be experiencing a short-term problem. Occasionally, Apple Servers may experience issues and go down, preventing the operation of linked services. If this is the situation, Apple fixes it right away. You have no choice but to wait. Visit the Apple System Status website to view the status of Apple servers. These are thing to be done when apple payment not completed