On the page Wiki>Main>Rockbox Utility, it says:
Download
Rockbox Utility is available for Windows, Mac OS X and Linux. Please note the following:
Older versions of Windows (e.g. 9X, ME) are not supported -- Rockbox Utility uses some windows functionality that has been added starting with 2000. Rockbox Utility runs on Windows 2000, XP, Vista, and 7.
Anyway, for some reason it didn't work on the computer with Windows 10, maybe for some other reason.
Thanks Jim