I've installed bstudio-sass and copied the path to the settings. My path is not the standard, but that shouldn't matter, should it? (it's: /Users/mh/.npm-global/bin/bstudio-sass) Running Bootstrap Studio 4.1.4 on Mac (High Sierra).
Turns out it does matter. I removed Node completely, re-installed with brew (no need for a custom prefix), then installed bstudio-sass and it all works.