Please try these steps if you are getting this error message:
1. Try rebooting the computer.
Go to the Windows Start Menu and select the Power icon, click Restart. Once the PC is booted up again, go into Steam and see if you still have the issue.
2. Delete Steam's download cache.
Open Steam Settings, go to Downloads...Clear Cache
3. Please verify the integrity of the game files on Steam
From the Steam Library, right-click iRacing and choose Properties.
Then click Installed Files
4. Repair the Steam Library folder.
In Steam, go to Settings...Storage...Repair Library
5. Reinstall iRacing in Steam.
First open Windows Task Manager, in the Processes tab, in the Background Processes section, right-click "iRacing.com Helper Service" and choose End Task.
Log into Steam and uninstall iRacing from there.
Then open Windows File Explorer and navigate to where you have your games installed (by default this is C:/Program Files (x86)/steam/steamapps/common) and delete the iRacing folder there.
Then log back into Steam and reinstall iRacing.
6. Run Steam as an administrator
Close Steam completely, then right click the Steam icon and select Run as Administator.