错误插入新定理

Question

\documentclass[a4paper,12pt]{book}



\usepackage[spanish]{babel}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{array}
\usepackage{bm}
\usepackage{booktabs}
\usepackage{boxedminipage}
\usepackage{caption}
\usepackage{cancel}
\usepackage{changepage}
\usepackage{cite}
\usepackage{color}
\usepackage{easylist}
\usepackage{esint}
\usepackage{eucal}
\usepackage{enumerate}
\usepackage{fancyhdr}
\usepackage{fontenc}
\usepackage{float}
\usepackage[a4paper]{geometry}
\usepackage{glossaries}
\usepackage{graphicx}
\usepackage{latexsym}
\usepackage{listings}
\usepackage{mathptmx}
\usepackage{mathrsfs}
\usepackage{microtype}
\usepackage{multicol}
\usepackage{multirow}
\usepackage{paralist}
\usepackage{pdfpages}
\usepackage{rotating}
\usepackage{setspace}
\usepackage{showkeys}
\usepackage{showidx}
\usepackage{subfiles}
\usepackage{subcaption}
\usepackage{syntonly}
\usepackage{textcomp}
\usepackage{theorem}
\usepackage{todonotes}
\usepackage{siunitx}
\usepackage{ulem}
\usepackage{url}
\usepackage{verbatim}
\usepackage{wrapfig}
\usepackage[utf8]{inputenc}
\usepackage{cleveref}
\usepackage[colorlinks=false, pdfborder={0 0 0}]{hyperref}

\newtheorem{theorem}{Teorema}

\begin{document}




 \part{TEORIA}

 \begin{theorem}
 Si a, b i c $=0$
 \end{theorem}
 \part{PROBLEMES} 
 \end{document}

错误如下：

仅可用于序言... \newtheorem 环境 thm 未定义... \begin{thm} \begin{document} 以 \end{thm} 结束.. \end{thm}