There was an issue with .net when patch 9 first released, though I thought that had been fixed. But if your game is definitely fully up-to-date you could try the workaround to that issue, which was to manually run the executable, which should be in your game files, eg:
C:\Program Files (x86)\Steam\steamapps\common\Baldurs Gate 3\DotNetCore\windowsdesktop-runtime-6.0.11-win-x64.exe
If yours is a different problem, though, perhaps if you could explain a bit more about what error message you're seeing and when, then someone else might recognise the issue as one they've come across and be able to help.
