RequirePackage{luatex85}
documentclass[border=1cm]{standalone}
usepackage{siunitx}

begin{document}
Time si{us}
end{document}

RequirePackage{luatex85}
documentclass[border=1cm]{standalone}
usepackage{mathtools}
usepackage[tuenc,no-math]{fontspec}
usepackage{unicode-math}
usepackage{siunitx}

begin{document}
Time si{us}
end{document}