This error typically means there is something wrong with a car file. If it's only happening with one particular car, you can navigate to C:\Program Files (x86)\iRacing\Cars and delete that one problem car. 

Then go back to iRacing and redownload and install.

If that doesn't work, go back and delete that one problem car and also the cars.dat and version.txt file in the Cars directory.

If it's happening with all cars, delete the entire cars folder and reinstall all of the cars.