I ran into this as well. To work around it, run these commands from within your theme directory:
./vendor/bin/sage meta
./vendor/bin/sage config
./vendor/bin/sage preset
meta allows you to specify the metadata for your theme (the name, etc., that goes in style.css).
config allows you to specify your theme’s dev URL and theme directory.
preset allows you to set up the theme with one of the supported frameworks (Bootstrap, etc.) and Font Awesome.