I have the 2017 downloadable version. I can't get it to open on my MacBook Air.

The 2017 download can only be installed on full desktop or laptop computer, Win or Mac.

See this TurboTax support FAQ for the Mac minimum system requirements - https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-mac-software/01/25609

See this TurboTax support FAQ for the Windows minimum system requirements - https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-windows-software/01/26175

The TurboTax online editions are only for tax year 2019 

You would need to contact TurboTax support for assistance in getting a Windows installation file pushed to your download account at no charge.

Support is open from 5am to 5pm Pacific time, Monday thru Friday (closed New Year's Day)

Use this website to contact TurboTax support during business hours - https://support.turbotax.intuit.com/contact/
Support can also be reached by messaging them on these pages https://www.facebook.com/turbotax/ and https://twitter.com/TeamTurboTax