If you’re certain that an app you want to install is from a trustworthy source and hasn’t been tampered with, you can temporarily override your Mac security settings and open it.
In the Finder, Control-click the app, choose Open from the menu, and in the dialog that appears, click Open. Enter your admin name and password when prompted. https://support.apple.com/en-us/HT202491
Very helpful, until it wasn't.
These instructions did allow me to install TurboTax Premier 2018 after a lot of frustration. But, the first thing the software asks is to find the tax file to import. "we support .tax2017". When I select "Find your tax return" the file I just downloaded from TurboTax.com is greyed out and not selectable because it's ".tax2018". Are you kidding? And the best part, customer service is closed on weekends, the time I assume most people are working on taxes?
Ok ... what exactly are you trying to do ?
After you install the 2018 program you can either transfer the .tax2017 file to start a new 2018 return OR open the .tax2018 file .... you cannot transfer the 2018 file.
First you must download the .tax2017 data file. Then start a NEW 2018 tax return. At the "Begin" screen it will as if you want to transfer from 2017. If TurboTax does not find the .tax2017 file on its own then use the find file button and locate and select the .tax2017 data file.
If it is grayed out then probably yiur user is not an administrator user (which is required) or the .tax2017 file does not have R/W (Read -Write) permissions of everyone. Select the file in the Finder and do a File -> Get Info and scroll down to permissions and change if necessary.