2018 TurboTax desktop software for Mac (CD/download) requires OS X v. 10.12 or higher.
https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software
If you purchased the desktop product and can't use the software, we can tell you how to get a refund. You can still use Online TurboTax since it is done through a browser.
If you have the CD, it also has the Windows version on it, if you happen to have a PC available, that uses Windows 7 SP1 or later.
