These are triggered if some of your content (specifically a car) are not up to date, did not update properly, or have become corrupt. You will need to remove and reinstall the pieces that are in error.
1. Go to your installation folder (generally C:\Program Files (x86)\iRacing).
2. Go to the "cars" directory.
3. Delete the "cars.dat" and "version.txt" files.
4. Delete the "safety" directory (it has the pace car).
5. Delete the directory for the car you were trying to use (or all the cars in the session if it was a multi-class session).
6. Refresh the web site and it will tell you that you have updates available.
7. Install the (now) missing car(s) and the shared car data.