SIunitx error when using lighter weight

Provide a definition for the missing command:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[defaultfam,light,scale=0.85]{montserrat}
\usepackage{siunitx}
\sisetup{%
 mode = math, % Same error if mode = text is used
 detect-all, % When, uncommented, the error described below occurs
 %detect-weight,  % When, uncommented, the error described below occurs
}

\DeclareRobustCommand\lseries{\fontseries{l}\selectfont}

\begin{document}
This is the weight and family of text desired but when siunitx shows heavier weight: \SI{75}{\micro\metre}
\end{document}

You need to load siunitx with the option detect-mode. (Please see the bottom of p. 17 and the top of p. 18 of the user guide of the siunitx package for the differences between detect-mode, detect-weight, etc.)

\documentclass{article}
\usepackage{geometry} % optional
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[defaultfam,light,scale=0.85]{montserrat}
\usepackage{siunitx}
\sisetup{ mode = math , detect-mode }  

\begin{document}
This is the weight and family of text desired. siunitx now shows the correct, i.e, thin weight: \SI{75}{\micro\metre}
\end{document}