The downside of this is you would need to download the Windows 10 disk image (ISO) file from Microsoft. The other option is to in fact install Windows 10 in the Parallels virtual machine.

Either way, this would be the quickest and easiest way to get your a self contained macOS that'll use the Home/Business TurboTax you need. However the 'worst' case is you would install a 10.13 High Sierra Parallels virtual machine.and then once it's working you'd upgrade the virtual machine to Mojave or Catalina. I'm fairly sure Parallels on a 10.13 High Sierra machine would allow you to install directly a 10.14 or 10.15 Mojave or Catalina virtual Parallels machine directly. This would be installed via Parallels utilizing your physical Macs Recovery Partition. The easiest method would be on your 10.13 High Sierra iMac w/ Parallels to install a self-contained copy of macOS. If you installed Parallels you'd could run a copy of Windows 10 inside it w/ the Windows version of TurboTax, or you could run macOS inside it with the Mac version of Turbotax. Your thoughts, advice and guidance would be very much appreciated! Thank you.

It is a dilemma! Unless Intuit makes it possible to use High Sierra for its 2020 TurboTax version ( which seems doubtful, I'm told at this time,) or the new Apple Silicon becomes available before the end of 2020 or very early 2021. It may be only a few months down the road, but I will likely miss the tax filing deadline if I want to use TurboTax- which I have been comfortably using for many years and do not want to switch to some other program. while I'm entirely ready, willing and able to purchase a new (2020) iMac with the Catalina O/S, I would VERY much prefer waiting a bit for the upcoming Apple Silicon line, with its new Apple chip and probably a larger than 27' display. My ( perfectly functioning) 2012 iMac 27' is NOT UPGRADABLE beyond High Sierra, which I presently use.

I've been informed by the folks at Intuit that I will NOT be able to do my 2020 taxes on my iMac using TurboTax unless I upgrade my O/S to Mojave or Catalina.

Turbotax for mac 2014 download