I am getting an error that the Authentication Failed. How to fix this? The game does not even start anymore. I uninstalled and reinstalled it and it still does not work.

Ubisoft needs to just die at this point. The games they make now are trash and they can't bother to optimize the handful of good games in their company's history. This game's campaign and co op modes were top tier in the Xbox 360 days and the fact that we live in an age of supercomputers that suffer running a game like this is laughable.

You have to go into the games files and delete a file for it to be able to run at all (at least on Windows 11):
steamapps\common\SplinterCellConviction\src\system and delete 'systemdetection.dll'