It is not an error. The warning happens in cases such as \section{Text with $m+a+t+h$} (a section title with math in it). You can use \textorpdfstring or ignore the warning.
– egregApr 30 '19 at 08:59
2
Might be an idea to edit the title to english and something more related to the question
– daleifApr 30 '19 at 09:14
2
Welcome to TeX S.E. Please provide a MWE. Btw, si vous êtes plus à l’aise pour poser vos questions en français, vous pouvez le faire sur texnique.fr
– sztruksApr 30 '19 at 10:08
\section{Text with $m+a+t+h$}(a section title with math in it). You can use\textorpdfstringor ignore the warning. – egreg Apr 30 '19 at 08:59