Has the game ever run okay on this computer? Or never?
IF NEVER:
Please go to C:\Program Files (x86)\Steam\steamapps\common\Crusader Kings II\ _CommonRedist\DirectX\Jun2010\ and run dxsetup.exe
Please go to C:\Program Files (x86)\Steam\steamapps\common\Crusader Kings II\ _CommonRedist\vcredist\2010\ and run vcredist_x86.exe and vcredist_x64.exe
Add ck2game.exe to the exceptions list/whitelist of your antivirus app, whatever you use.
Also if you have installed a gaming VPN app called Evolve, or the MSI Gaming App, or Duet, disable it as it clashes with this game.
IF IT USED TO WORK:
DXDIAG is a program you run from a command prompt or the Windows start menu 'run' dialog box (or 'search programs' in Windows 7 or later). After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.
Please attach your Documents/Paradox/CK2/settings.txt.
From your Documents/Paradox/CK2/logs/ folder, attach system.log .