The Error Code 6 means that there was an issue communicating with the Google Play app on the phone (various possible reasons).

1) Reboot the device and try the purchase again

2) Make sure that the Google Play app is up to date - Open Google Play > tap the side menu button > tap Settings > tap on "Build version" to have it check that you have the latest (more info). Then try the purchase again.

If neither of the steps above solve the issue, please contact us.