I initially had Garmin Express stating I was on latest version, even though I was on 2020 version and 2021 version was available.
I attempted again a few days later (new initial USB file from car sat nav but without reformatting USB drive) and it accepted the 2021 update and installed OK.
I used a SanDisk 16GB USB2 flash drive.
There was also an update to Garmin Express app, at least on my Mac version, which I'd applied.