Source Sans Pro is included with Dorico, because it’s used as the main user interface on macOS, and is used in a number of places on Windows as well (though the default UI font on Windows is Segoe UI, because that looks better on crummy low pixel density displays than Source Sans Pro).
I’ve chatted with a colleague and we’re not quite sure why these fonts would be missing on your system, but what we recommend is that you download the fonts using the link below:
SourceSansPro.zip (1.0 MB)
Unzip this archive, and then drag the fonts into the Fonts folder in Windows Explorer to install them. That should take care of the problem.