Especially if you are on Windows 7, you must make sure that you have sp1 installed. https://support.microsoft.com/en-us/help/15090/windows-7-install-service-pack-1-sp1
After that, please navigate to your C:\Program Files (x86)\iRacing\msvc folder (or wherever you have iRacing installed) and run the file vcredist_vc2015_x64.exe
then try and launch the sim again.
If you are still having this issue:
In the same location and run the other 5 .exe files. If you are prompted to Repair, Uninstall or Close, choose Close (and then Yes, you are sure you want to cancel).If you are still having this issue, if you have the link to the iRacing UI on your desktop, or in the Start menu, please try and launch iRacing that way. Otherwise, you can navigate to the installation folder (generally C:\Program Files (x86)\iRacing), and double-click on iRacingSim64DX11.exe.