Update: 9/24/2021 - 11:15 AM CST -
It looks like the issue with World War Z: Aftermath has been resolved, as the game is currently available and no longer listed as an issue on the Status page.
According to GeForce Now's Status page, World War Z: Aftermath is currently unavailable to play as it is in Maintenance Mode. This is due to an issue that was found, and NVIDIA is working to resolve the issue and get the game back online.
We will update this article if any new information is posted by NVIDIA.
World War Z: Aftermath is in maintenance mode
Investigating - World War Z: Aftermath is in maintenance mode due to an issue we found today. We are working to resolve it and make the game available to play again.
Sep 21, 11:59 PDT