TurboTax 2020 requires Mojave (or later). If you have a Mac on which Apple does not support Mojave, there is something that you can do, but it requires some technical expertise and is a little bit risky. The web page http://dosdude1.com/mojave/ describes the (free) "macOS Mojave Patcher Tool for Unsupported Macs", lists the Mac models on which the patcher will work, and provides links to download the patcher and Mojave. If I were doing this, I'd create a separate partition and install the patched Mojave in a separate boot partition.
(There is also a (free) "macOS Catalina Patcher" tool for unsupported Macs at http://dosdude1.com/catalina/ )
The biggest downside of this is that Apple won't provide any support for you running Mojave in this way.
