Sometimes the game's executable or a critical dependency is just corrupted.
This is the most common culprit. Sonic Generations is very sensitive to privilege levels. How To Fix Sonic Generations Steam Client Is Not Running
In this guide, we will walk through 11 proven methods to fix the "Steam Client Is Not Running" error, ranging from a simple restart to advanced DLL modifications. Sometimes the game's executable or a critical dependency
Why this works: The wrapper loads before the DRM, telling Steam the game is already running, effectively skipping the "client not running" check. How To Fix Sonic Generations Steam Client Is Not Running
Overlays from Discord, GeForce Experience, or even Steam itself can confuse the DRM.