您正在将 fi 输入为 unicode 字符 U+FB01。请将其替换为“fi”,或使用 inputenc 并定义合适的替换:
\documentclass[12pt,a4paper,oneside]{article}
\usepackage[utf8]{inputenc}
\DeclareUnicodeCharacter{FB01}{fi}
\begin{document} \thispagestyle{empty}
\section{Introduction} connectivity, which can be either X-DSL or fiber.
\end{document}