\section coming up as undefined when using sectsty

Question

I have a style requirement for headings. Header 1 = 16pt bold, Header 2 = 14pt bold underline. Font is required to be Palatino Linotype. I got this document shell from a classmate and have only made one real document in latex. I couldn't figure out how to modify headers without using the sectsty package, but when using sectsty I am getting "Undefined control sequence errors for my \section, \subsection, \tableofcontents and \begin but the formatting is correct. It's not messing the document up but I'd like to figure out why this is happening.

My sections aren't complicated(ex: /Section{Introduction})

\documentclass[11pt,titlepage]{article}
\usepackage{float}
\usepackage{graphicx, fancyvrb, natbib, setspace}
\usepackage{listings}
\usepackage[usenames,dvipsnames]{color}
\usepackage[usenames]{color}
\usepackage{geometry}
\usepackage{amsmath}
\usepackage{multirow}
\usepackage{caption}
\usepackage[T1]{fontenc}
\usepackage{textcomp}
\usepackage{mathpazo}
\usepackage{sectsty}
\usepackage{fncychap}
\usepackage[normalem]{ulem}
\captionsetup[table]{font=bf}
\subsectionfont{\Large\PalatinoLinotype\underline}
\sectionfont{\LARGE\PalatinoLinotype}

Answer 1

If Palatino is required for the whole document, you're already loading mathpazo to have Palatino in both text and math mode.

MWE:

\documentclass[11pt,titlepage]{article}

\usepackage{mathpazo}

\usepackage{sectsty}
\subsectionfont{\Large\underline}
\sectionfont{\LARGE}

\usepackage{lipsum} % just for the example

\begin{document}
\section{A section}
\lipsum[1]
\[a+b=c\]
\subsection{A subsection}
\lipsum[2]
\[x+y=z\]
\end{document}