There is a fix for this coming up but for now find Documents\CrewChiefV4\Profiles\defaultSettings.json and open it with Notepad or another text editor. Look for
Code:
"run_immediately": true,
and change it to false and save the file.
Now when you start Crew Chief you should be able to select the game you want. If you want to run Assetto Corsa open the Properties window and search for ACS install path and set it to C:\Program Files (x86)\Steam\steamapps\common\assettocorsa or wherever you have it.