My Apple Mini is too old to use Sierra. Can I get a version that will work with El Capitan as last year?

[mesquitebean]

2018 TurboTax cannot install on El Capitan (OS X 10.11)   Here are the minimum system requirements for the 2018 desktop software on a Mac, which requires OS X 10.12 or higher:

https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software

You can use Online TurboTax on the older computer, since the web version is accessed through a browser.  But the Online version is not as full-featured as the desktop version, and some online editions have form limitations.  And there is no Forms Mode in Online TurboTax for direct viewing and editing of forms during preparation.