I had the same problem -- Specifically: unable to open TTPremier 2020 and/or 2019 on Mac OS Big Sur. The two steps recommended above by jwhaskell1 and audge66 seem to have finally resolved the problem (at least thus far):
1)" from Finder use GO tab at top then GO To Folder ~/library then PREFERENCES then delete the file named "com.intuit.turbotax.2020.plist" and
2) launch TT2020 but disregard Intuit's error message (i.e. close the box without sending Intuit a crash report!)
I repeated the same steps for TT2019 (i.e., deleting "com.intuit.turbotax.2019.plist"
With those two steps I am finally able to run both editions of TT. Thank goodness!
