Found my old note. Sometimes the .tax file downloads as a zip file.
Can you just take the zip off the end of the file? Did you use Microsoft Edge to download it? Try using a different browser. Post on a .tax file downloading as a zip file, see the last 2 posts
https://ttlc.intuit.com/community/after-you-file/discussion/how-to-recover-tax2018-file/00/1261004
