This is the most common trigger. If Sonic Generations is running as an Administrator, but Steam is running as a standard user (or vice versa), Windows creates a "session barrier." The game literally cannot see the Steam client running in the other permission level.
Before trying advanced fixes, clear any minor software glitches that might be blocking the connection between the game and the Steam client. This is the most common trigger
A: Uninstall all mods temporarily. Go to the Sonic Generations folder and delete d3d9.dll or dsound.dll if present. Mod loaders sometimes inject code that confuses the Steam authentication. This is the most common trigger
This article dives deep into why this error occurs and provides a definitive, step-by-step guide to fixing Sonic Generations once and for all. This is the most common trigger