The TurboTax software for tax year 2018 requires Mac OS X v.10.12 or later (64-bit required)
See this TurboTax support FAQ for Mac minimum system requirements - https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software
If you cannot upgrade or do not want to upgrade your Mac OS then you can request a refund of the software purchased from TurboTax using this website - https://support.turbotax.intuit.com/forms/refund-request.jsp
You can use the TurboTax online editions - https://turbotax.intuit.com/personal-taxes/online/
