So I got this error today working on a MacBook running MacOS Mojave trying to open a TurboTax for Mac 2019 file in TurboTax for Mac 2020. Rather than the software uninstall/reinstall trick, instead I focused on why the file might have been corrupt. Turns out I was trying to open my 2019 Tax file from Dropbox where I have the "SmartSync" feature turned ON to save space on my hard drive. Dropbox SmartSync doesn't download the file until you need it. This feature doesn't appear to work through the File--Open request made by the TurboTax program. So I opened a Mac Finder window, used the two finder click on the file I wanted to open in the Finder to bring up the Mac file menu, and the DropBox Features menu pops up. Where it says "SmartSync" you move the pointer over to the "Local" field and just click on the word Local. It will then fully download the file to your Hard Drive. Now the file is ready to be opened by TurboTax. Voila.
