Please open your Windows Task Manager by right-clicking on your Windows Start Button (at the bottom left of your screen) and choose Task Manager.
If in the Apps section you see iRacing.com Simulator listed right-click on it and choose End Task.
If you scroll down in the Background processes you see anything iRacing related OTHER THAN iRacing.com Helper Service, do the same (right click and choose End Task).
iRacing.com Helper Service should remain running at all times.
Another possible reason that the iRacing simulation is unable to close successfully is due to a conflict with version 2020.6.58918 of Logitech G HUB software, specifically hardware lighting effects.