As it’s a .liquid file It’s likely is has liquid syntax in the file that a SASS converter cannot read as it’s not really SASS until after the file is rendered to the frontend.
Open your website and view the source html (ctrl+u) find the link to the theme.scss.css file.
If you cannot simply use that CSS then with that url remove the .css part and that should give you a url to the theme.scss file that has the raw rendered SASS file in scss syntax.