\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[natbibapa]{apacite}
\title{For testing}
\begin{document}
Information about natbib in \citeauthor{sharelatex}.
\bibliographystyle{apacite}
\bibliography{bibli.bib}
\end{document}
This gives the error "Missing $ inserted". What has mathmode to do with apacite or citeauthor?
$
in yourbibli.bib
file..bib
entry (e.g., underscores) could cause this. But without looking at the specify.bib
entry, it's hard to tell.