I tried starting TurboTax 2020, 2019, and 2018, and all required an update which then failed with the error download failed message. I tried on a Mac running Big Sur, and one running Mojave, with the same result. I tried it with the home wifi and using my phone as a hotspot. Each time the download would get to a slightly different number of megabytes downloaded before it failed.
Finally, I timed the process with a stopwatch. Every time I ran it, it would fail a few seconds more than one minute after the megabytes downloaded progress numbers started. The total number of megabytes downloaded varied a bit, but the time it failed was always exactly the same.
My phone hotspot is slightly faster than my home Internet, and with that the download was very close to finishing in that one minute. After repeated tries, sometimes it was able to complete the updates. As long as it finished before 60 seconds after the progress numbers started, the update worked. My home Internet, was too slow to ever get done in time, the phone hotspot was just on the edge.
As far as I can tell the workaround for this bug is to be on a faster Internet connection that allows the update to complete within a minute.
