I am assuming you've already checked the obvious things: you're in the right game mode, you've got the Source SDK, etc. Please read the game specific sections and search the forums for information how to set-up the configuration.

If you're getting errors related to paths, i.e. "can't find ...", go to the  Configuration  >  Games  >  HL2  AND  Configuration  >  Games  >  Steam , and make sure your paths are set correctly.

1. Make sure you're using the latest version of QuArK.
2. Make sure you've got a CLEAN install of QuArK.
3. Make sure your game-files (and Source SDK!) in Steam are up-to-date.
4. Make sure Steam is up-to-date.
5. Run the Source SDK (SDKLauncher.exe). This will check the integrity of the Source SDK files, and move updated files to their proper location.
6. Can you run and play the game itself? Can you run the Hammer editor?
7. Does QuArK work correctly in other game modes?

If the error is coming from QuArKSAS.exe, please go to *Steam*/steamapps/*accountname*/sourcesdk/bin and check the QuArKSAS.log file (the one produced right after the crash).