Try doing a clean boot and then see if you can start the game. Start/restart your Mac and hold the Shift key down as soon as the progress bar comes up in the startup window, releasing it after the desktop appears. Alternately, from the login window, hold the Shift key when you click the Log In button, and release it when you see the Dock.
Does it make a difference if you exit out of Steam and start the game from the app container? In the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Browse Local Files...' button, then exit out of the Steam client and try launching the game directly from Finder.
Try using the Disk utility to check your drive(s).
Try manually creating a 'Larian Studios' folder in your '~/Documents' folder and then extract the replacement 'Divinity Original Sin 2 Definitive Edition' there from
this download. If the problem is in creating the folder, this may help, but if the game is crashing before getting to the point it would be created, it would not make a difference.
The graphicSettings.lsx file is set to 1280x720 Windowed mode, Very Low quality preset and low audio quality, which you can change in the options (manually, or hit autodetect) if this gets the game to start.
If that doesn't help, please check if any crash reports are being created (collected in the 'Console' application, under 'User Reports'; alternately, the crash reports may be saved in the '~/Library/Logs/DiagnosticReports' or '~/Library/Logs/CrashReporter' folders), and email one to supportdos2@larian.com or post/PM a Dropbox link, etc.
