Its not another company's tax return its turbo tax only and file has .taxbak extension itself that's the file generated automatically when I submitted return. I...

Ok, but i have .pdf that I have saved when I submit my returns. Even i tried to renamed it with .tax2019 and tried to open but didnt work either.

No the one I have is .taxback, I have tried to rename file but then I was getting error "We are having trouble opening your file. You can try using your back-up...

When I am opening my tax file I am getting error message "We do not support opening of the turbotax return files older than 2018. Please choose 2019 or 2018 tax...