\documentclass[12pt, titlepage, oneside, a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[english]{babel}
\usepackage{setspace}
\usepackage{amssymb, graphicx, fancyheadings}
\addtolength{\textheight}{20mm}
\addtolength{\voffset}{-5mm}
\renewcommand{\sectionmark}[1]{{#1}}

\begin{document}
	\begin{titlepage}
		\thispagestyle{empty}
	\end{titlepage}

	\newpage

	\pagenumbering{roman}
	\tableofcontents
	
	\newpage

	\pagenumbering{arabic}

\section{Introduction}
% THIS WORKS:
A\textsuperscript{b} 

% THIS DOESN'T WORK:
A\textsubscript{b}

\end{document}