To reboot your apple TV:
Hold the Menu and Down buttons on your Apple TV remote for six seconds. After the Apple TV restarts, try Netflix to see if the issue was resolved.
You can also restart your Apple TV by unplugging it from the power source for sixty seconds and plug it back in to the outlet.
According to Netflix support, the 789 message indicates that the device being used to watch online streaming content is not compatible with the service that it is subscribed to.
If restarting your Apple TV did not clear the 789 error, you may need to contact Netflix. They are aware of the problem.