Please try and install the EasyAntiCheat software. You can do this by going to C:\Program Files (x86)\iRacing\EasyAntiCheat (or C:\Program Files (x86)\Steam\steamapps\common\iracing if you purchased and installed through Steam) and run the set up file from there.

If after that you are still having this issue, please look in your Documents/iRacing folder and see if you have a file in there called sim_launch.txt. If you do, please send that to support@iracing.com. Also, look in your Documents/iRacing/logs folder and send us any log files that are in there.