Appendices labelled with numbers instead of letters, and throuwing errors

Question

I'm trying to add an appendix to my document but when I do, the appendices are labelled with numbers instead of letters, and I get the following errors:

LaTeX Error: No counter 'chapter' defined.
LaTeX Error: Command \@chapapp undefined.
LaTeX Error: Command \thechapter undefined.

I am using the following code:

\documentclass[12pt]{article}
\usepackage[english]{babel}
\usepackage[letterpaper,top=2cm,bottom=2cm,left=3cm,right=3cm,marginparwidth=1.75cm]{geometry}
% Useful packages
\usepackage[utf8]{inputenc}
\usepackage{float}
\usepackage{eurosym}
\usepackage{array}
\usepackage{textcomp}
\usepackage[round, numbers]{natbib}
\usepackage{graphicx}                   % graphics package
\graphicspath{ {figures/} }% specify the path where figures are located
\usepackage{epstopdf}                   % converts eps files to pdf
\usepackage{fancyhdr}                   % fancy headers and footers
\usepackage{url}                        % nicely format url breaks
\usepackage{relsize}                    % font sizing hierarchy
\usepackage{booktabs}                   % professional looking tables
\usepackage[config, labelfont={bf}]{caption,subfig}
\usepackage{mathrsfs}  % additional math scripts
\usepackage[Bjarne]{fncychap}   % creates fancier chapter titles
\usepackage{setspace}
\usepackage{amsmath}
\usepackage{xcolor}
\usepackage{soul}
\usepackage{dirtytalk}
\usepackage{multicol}
\usepackage{tabto}
\usepackage[toc]{appendix}
\numberwithin{equation}{section}
\usepackage{mathrsfs}
\usepackage{chngcntr}
\counterwithin{table}{section}
\counterwithin{figure}{section}
\usepackage[justification=centering]{caption}
\setlength{\parskip}{0.5em}
\usepackage{rotating, graphicx}
\usepackage{enumitem}
\setlist[description]{style=nextline}
\setlength{\belowcaptionskip}{-15pt}
\renewcommand{\arraystretch}{1.5}
\usepackage{biblatex} %Imports biblatex package
\addbibresource{bibliography.bib} %Import the bibliography file
% \renewcommand{\baselinestretch}{1.5}
% \onehalfspacing      % line spacing
\spacing{3}
\geometry{
    a4paper,
    left = 1.0in,
    right = 1.0in,
    top = 1.0in,
    bottom = 1.375in
}
\begin{document}
\section{Section 1}
blah blah blah
\appendix
\section{Appendix}
blah blah blah
\end{document}

Sorry I know this isn't really "minimum example" but i suspect there is some conflict with one of the packages that is causing this to occur. I'm new to LaTex and inherited this preamble from a lecturer.

Thank you!!

Answer 1

You are using the article class, together with the fncychap package. That makes no sense, since that package was made for classes which define chapters. That's not the case of the article class.