If you receive an error that Steam must be running when attempting to launch one of our titles, this is indicating that the game client is having trouble connecting to Steam. The following suggestions should fix this for most users:
- Launch Steam with Administrator privileges
- Set the Steam.exe or shortcut to launch as an administrator and restart Steam. If Steam is set to auto launch with Windows, make sure the .exe is set.
If this does not help you may need to try a Manual Steam File refresh. Instructions for that can be found here: https://support.killingfloor2.com/en/steampc-knowledgebase/article/manual-steam-file-refresh