Here is a checklist of solutions, ordered from easiest to most complex, to resolve the issue.
A failed update or interrupted download can leave the "stub" of the game visible in your library without the actual files behind it. Step-by-Step Fixes for ATC4 1. Launch Directly from the Steam Library I Am An Air Traffic Controller 4 Application Not Found
If you're staring at an "Application Not Found" or a similar start-up error, these steps are the most effective solutions gathered from the community and TechnoBrain FAQ . Here is a checklist of solutions, ordered from