I found out too late that my 2008 Mac is not compatible with my Turbotax download. I can't upgrade my OS to Mojave which is what is required.Is there anything I can do?

You can call and ask for the Windows download.  Contact Customer Service
5am-5pm Pacific Time
Monday-Friday
https://support.turbotax.intuit.com/contact/

What is the Turbo Tax phone number
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632

Window System Requirements

https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-windows-software/00/26175

Two things:

1) Call customer support and have them switch you to a Windows download.

2) Ask support to switch switch you to the online version that can b used with any compatible web browser.

Here is a TurboTax FAQ for contacting customer support.

https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number