In this MWE, I can't have € in math mode since I've updated siunitx
package.
Before it worked fine.
I found that take the fourier
package off make it work again
\documentclass[border=3pt]{standalone}
\RequirePackage{fourier}
\RequirePackage{fontspec}
\setmainfont{Erewhon}[
Extension=.otf,
UprightFont=*-Regular,
ItalicFont=*-Italic,
BoldFont=*-Bold,
BoldItalicFont=*-BoldItalic,
SlantedFont=*-RegularSlanted,
BoldSlantedFont=*-BoldSlanted,
SmallCapsFont=*-SmallCaps,
SmallCapsFont=*-Regular,
SmallCapsFeatures={Letters=SmallCaps}
]
\usepackage{siunitx}
\sisetup{
locale=FR,
detect-all,
inter-unit-product = \ensuremath{{}\cdot{}},
group-minimum-digits=4}
\begin{document}
\SI{3}{€} % works
$\SI{3}{€}$ % works only without fourier
\end{document}
Best Answer
Some time ago I flipped the standard setting for units from text to math mode. You can reverse this by setting
unit-mode = text
, for example