Error 491 can jump when you try to download an application or game from the Android PlayStore.
The reason for the error is that the system does not recognize your Google account. To fix it go to Settings -> Applications and search Google PlayStore, access the application info and select «delete data" and "clear cache«. Restart your device and try to download the application again.
This is the quick fix, but if you find that it doesn't work, remove your Google account from your phone or tablet, restart your device, and add the account again. You shouldn't have a bigger problem.