Currently iRacing runs only on the Windows Operating System (Windows 7 sp1, 8, 8.1 and 10).
You can find our system requirements here: https://www.iracing.com/membership/system-requirements/
If you have a Mac, there is still the Boot Camp option for Mac users. Learn more here: https://support.apple.com/boot-camp where you can install a Windows partition on your Mac and then run iRacing from that partition.