Error in compilation

Load the package wasysym after the amspackages ( amsthm in your case)

\documentclass[10pt]{book}

\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{blindtext}
\usepackage{siunitx}
\sisetup{output-decimal-marker = {,}}

\usepackage[a5paper]{geometry}
%\usepackage[a5paper, top=2in, bottom=1.5in, left=1in, right=1in]{geometry}
\usepackage{scrextend}
\changefontsizes{10.0pt}  % Change base font size

\usepackage{lettrine}
\usepackage{fancyhdr}

\pagestyle{fancy}
\fancyhf{} % clear all fields
\fancyhead[RO,LE]{\thepage}
\linespread{1.3}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}

\newtheorem{thm}{Theorem}[section]
\newtheorem{lem}[thm]{Lemma}
\newtheorem{prop}[thm]{Proposition}

\usepackage{wasysym}

\begin{document}
\tableofcontents
\newpage

\section*{section}
demo
\end{document}

\usepackage{savesym}
\savesymbol{iint}
\usepackage{wasysym}
\restoresymbol{WAS}{iint}