Why can't I use \bullet without getting a syntax error?

The \bullet command is defined for use in math mode, so you need to write $\bullet$ (or \(\bullet\)) to use this command within your text. If you use \bullet outside math mode it will generate the error Missing $ inserted.

Alternatively, the \textbullet command can be used outside of math mode with no need for dollar signs or \(...\), as demonstrated in the following example:

\documentclass{article}
\title{A random walk in \LaTeX}
\author{Overleaf Team}
\begin{document}
\maketitle
\section{In the beginning...}
This example demonstrates using \verb|\bullet| in math mode, producing \(\bullet\) and \verb|\textbullet| outside math mode, also producing \textbullet.
\end{document}

 Open this example in Overleaf

This example produces the following output: