The most reliable way to get this file back is through rather than downloading random files from the internet, which can carry security risks. Step 1: Restore the File via Steam (Recommended)
If verifying doesn't work, your antivirus might be "eating" the file as soon as it's downloaded. Open your antivirus settings (e.g., Windows Security). Check the or Quarantine section. If you see steam_api.dll or sonic2app.exe , select Restore . steam apidll sonic adventure 2 upd link