By default SILE uses the font Gentium Plus to render all documents that do not specifically call for something else. If this font is not available on your system you may encounter errors. Of lower priority depending on your use case, the math typesetting package defaults to using Libertinus Math and the default monospace font is Hack.
Homebrew does not supply any of these font dependencies in default casks, but they can be added by tapping cask-fonts:
brew tap homebrew/cask-fonts
brew install --cask font-gentium-plus
brew install --cask font-libertinus
brew install --cask font-hack
Alternatively you can download and install the fonts yourself:
https://software.sil.org/gentium/
https://github.com/alerque/libertinus
https://sourcefoundry.org/hack/