In the game launcher please set Display mode to windowed.
IF that doesn't help, Has the game ever run OK on this computer? Or never?
IF NEVER:
Install ALL these Microsoft modules:
www.microsoft.com
www.microsoft.com
This update is the latest in a cumulative series of feature additions and bug fixes for Microsoft Visual C++ 2015 Redistributable.
www.microsoft.com
The Microsoft DirectX® End-User Runtime installs a number of runtime libraries from the legacy DirectX SDK for some games that use D3DX9, D3DX10, D3DX11, XAudio 2.7, XInput 1.3, XACT, and/or Managed DirectX 1.1. Note that this package does not modify the DirectX Runtime installed on your Windows...
www.microsoft.com
(run dxsetup.exe after extracting from this download)
Add CK3.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
IF IT USED TO WORK:
Please do a full **clean** re-install:
- move any valued save games elsewhere
- "uninstall" in Steam-CK3
- manually delete both the Steam/SteamApps/common/CK3 AND Documents/Paradox/CK3 folders
- re-install game in Steam, run a Steam Verify when done.
- add CK3.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test
In particular make sure there are no files or folders left under either the Steam CK3 or Documents CK3 locations, before installing again.
If those don't help:
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/CK3/pdx_settings.txt .
From your Documents/Paradox/CK3/logs/ folder, attach system.log , error.log