Daniel O. solved the problem. Many thanks, Dan!
The solution was to remove the application from the Applications Folder, then go into the Library and delete every file and directory relating to TurboTax 2021, and finally download and install a new version.
If others encounter the same problem, please note: The information Daniel O. was kind enough to provide, headed "Remove and reinstall TurboTax for Mac," was dated July 27, 2021. I am running MacOS Big Sur (11.6). Consequently, two of the files weren't in the Library and one was in a different directory. Perhaps Intuit might consider updating that extremely useful page of information!
