If you are seeing this error, you are in the wrong app. We are seeing this with some people that played during Steam Next Fest and are trying to login to the "Stormgate Open Beta" app. The correct app is simply named "Stormgate". 

If you can't see the Stormgate app, here are some troubleshooting options you should try. These are unique, not sequential. 

• Make sure that your Steam Library is toggled for "home" and not just "downloads". 
• Log out of Steam and then back in again. This can refresh your library. 
• Clear your download caches by navigating to your Steam Settings > Downloads > Clear Cache.

• Move the install location on your hard drive by navigating to Steam Settings > Storage > ... > Repair Folder or Repair Library.
• Verify local files by navigating to your Steam Library. Right click Stormgate and select Properties. Select "Installed Files" and then click "Verify integrity of game files". 

• Reinstall Steam to the same location. (Steam installer here). Installing to the same location preserves all installed games. 

If none of the above solved the problem, there are more tips here: https://www.makeuseof.com/steam-not-recognizing-installed-games-fix/. Lastly, if all of these fail to force the Stormgate app to be visible, you are experiencing a Steam Library bug and will need to contact Steam Customer Support to get the app to appear.