Setting custom chapter numbering in Markdown - header

Question

I'm creating an R Markdown document which outputs a PDF document. My YAML header is the following:
---
title: "Introduction"
author: "John Doe
date: "August 26, 2018"
mainfont: Pancetta Pro
documentclass: book
output:
pdf_document:
number_sections: true
df_print: kable
fig_caption: yes
fig_width: 6
highlight: tango
includes:
in_header: preamble.tex
latex_engine: xelatex
geometry: headheight=25pt, tmargin=25mm, bmargin=20mm, innermargin=20mm, outermargin=20mm
---
In the preamble.tex file I want to have the following LaTeX commands (which just modify the way the headers are displayed):
\renewcommand{\chaptermark}[1]{\markright{#1}}
\renewcommand{\sectionmark}[1]{}
\renewcommand{\subsectionmark}[1]{}
\usepackage{titlesec}
\titleformat{\chapter}[hang]{\Huge}{\bfseries\thechapter}{0.2pt}{\thicklines\thehook}[\vspace{0.5em}]
However, when these last lines are included in the preamble.tex I get an error when knitting the R Markdown file:
! Argument of \paragraph has an extra }.
<inserted text>
\par
l.1290 \ttl#extract\paragraph
Error: Failed to compile Template.tex
I can't figure out why it won't run. The contents of thepreamble.texfile are the following:
% !TeX program = lualatex
\usepackage{relsize} % To make math slightly larger.
\newcommand{\thehook}{%
\hspace{.5em}%
\setlength{\unitlength}{1em}%
\raisebox{-.5em}{\begin{picture}(.4,1.7)
\put(0,0){\line(1,0){.2}}
\put(.2,0){\line(0,1){1.7}}
\put(.2,1.7){\line(1,0){.2}}
\end{picture}}%
\hspace{0.5em}%
} %This creates the "hook" symbol at the beginning of each chapter.
\usepackage{anyfontsize}
\usepackage{fontspec}
\setmainfont{Pancetta Pro}
% We set the font for the chapters:
\newfontfamily\chapterfont{Pancetta Pro}
% And now for the sections:
\newfontfamily\sectionfont{Pancetta Pro}
\usepackage{fancyhdr}
\fancyhead{}
\fancyfoot{}
\renewcommand{\headrulewidth}{0pt}
\renewcommand{\footrulewidth}{0pt}
\fancyhead[RO]{\large\sffamily\rightmark\thehook\textbf{\thepage}}
\fancyhead[LE]{\large\sffamily\textbf{\thepage}\thehook\rightmark}
\fancypagestyle{plain}{%
\fancyhf{}
}
\pagestyle{fancy}
\renewcommand{\chaptermark}[1]{\markright{#1}}
\renewcommand{\sectionmark}[1]{}
\renewcommand{\subsectionmark}[1]{}
\fontsize{12}{20}\selectfont
\usepackage{titlesec}
\titleformat{\chapter}[hang]{\Huge}{\bfseries\thechapter}{0.2pt}{\thicklines\thehook}[\vspace{0.5em}]
When excluding the last 6 lines in the previous code there's no error and the pdf is created.

Answer 1

If you want to use titlesec together with rmarkdown you have to add
subparagraph: yes
to your YAML headers, c.f. several other answers.
The default LaTeX class used by rmarkdown is article, which has no chapters. You should add
documentclass: report
or
documentclass: book
to your YAML header.