Error message converting from markdown to PDF, Package calc Error: \let ' invalid at this point

I am trying to convert a R markdown document to PDF. However, it fails to compile and gives me the following error message: ! Package calc Error: \let ' invalid at this point.

My document consists of many child documents and I can compile all of the child documents separately without an error occuring. I did not load the calc package in my YAML header and can't figure out where the error comes from.

I am new to latex and R markdown and would really appreciate any tips on how I can solve this!

Here is my R markdown code:

---
output:
pdf_document:
fig_caption: yes
number_sections: yes
bibliography: library.bib
csl: chicago-author-date.csl
urlcolor: black
fontsize: 12pt
geometry: margin = 1.2in
- \usepackage{placeins}
- \usepackage{fancyhdr}
- \usepackage{setspace}
- \usepackage{chngcntr}
- \usepackage{microtype}
- \usepackage{mathtools}
- \usepackage{amsmath,amssymb,bm}
- \usepackage{dsfont}
- \usepackage{pdflscape}
- \usepackage{booktabs}
- \usepackage{caption}
- \usepackage{subfig}
- \onehalfspacing
- \counterwithin{figure}{section}
- \counterwithin{table}{section}
---

---
nocite: |
@Beck2001, @Piketty2014, @Nickell2006, @LaPorta1997, @Quinn2008
...

{r global_options, include=FALSE}
knitr::opts_chunk\$set(fig.path = 'figures/',
echo = FALSE, warning = FALSE, message = FALSE)


\pagenumbering{gobble}

{r child = 'B.2-title_page.Rmd'}


\newpage

{r child = 'B.3-declaration.Rmd'}


\newpage
\pagestyle{fancy}

\renewcommand{\footrulewidth}{0pt}

\pagenumbering{roman}

\section*{Abstract}

{r child = 'B.4-abstract.Rmd'}


\newpage
\setcounter{tocdepth}{2}
\tableofcontents

\newpage
\pagenumbering{arabic}

\newpage

{r child = 'B.5-introduction.Rmd'}


\FloatBarrier
\newpage
\fancyhead[CO,CE]{Financial Deregulation and the 'Big Bangs'}

{r child = 'B.6-big_bang.Rmd'}


\FloatBarrier
\newpage

{r child = 'B.7-methodology.Rmd'}


\FloatBarrier
\newpage

{r child = 'B.8-main_results.Rmd'}


\FloatBarrier
\newpage

{r child = 'B.9-conclusion.Rmd'}


\FloatBarrier

\newpage
\listoffigures

\newpage
\listoftables

\newpage

{r child = 'B.10-appendix.Rmd'}


\FloatBarrier
\newpage

# References


Update So this is the .tex version of my markdown document that fails to convert to PDF. Rendering the document Example.Rmd by itself works fine. But included as a child document it produces the above error. Since I am an absolut beginner in markdown and Latex, maybe I'm not supposed to use \onehalfspacing the way I did?

\documentclass[]{article}
\usepackage{lmodern}
\usepackage{amssymb,amsmath}
\usepackage{ifxetex,ifluatex}
\usepackage{fixltx2e} % provides \textsubscript
\ifnum 0\ifxetex 1\fi\ifluatex 1\fi=0 % if pdftex
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\else % if luatex or xelatex
\ifxetex
\usepackage{mathspec}
\else
\usepackage{fontspec}
\fi
\defaultfontfeatures{Ligatures=TeX,Scale=MatchLowercase}
\fi
% use upquote if available, for straight quotes in verbatim environments
\IfFileExists{upquote.sty}{\usepackage{upquote}}{}
% use microtype if available
\IfFileExists{microtype.sty}{%
\usepackage{microtype}
\UseMicrotypeSet[protrusion]{basicmath} % disable protrusion for tt fonts
}{}
\usepackage[margin=1in]{geometry}
\usepackage{hyperref}
\hypersetup{unicode=true,
pdfborder={0 0 0},
\urlstyle{same}  % don't use monospace font for urls
\usepackage{graphicx,grffile}
\makeatletter
\def\maxwidth{\ifdim\Gin@nat@width>\linewidth\linewidth\else\Gin@nat@width\fi}
\def\maxheight{\ifdim\Gin@nat@height>\textheight\textheight\else\Gin@nat@height\fi}
\makeatother
% Scale images if necessary, so that they will not overflow the page
% margins by default, and it is still possible to overwrite the defaults
% using explicit options in \includegraphics[width, height, ...]{}
\setkeys{Gin}{width=\maxwidth,height=\maxheight,keepaspectratio}
\IfFileExists{parskip.sty}{%
\usepackage{parskip}
}{% else
\setlength{\parindent}{0pt}
\setlength{\parskip}{6pt plus 2pt minus 1pt}
}
\setlength{\emergencystretch}{3em}  % prevent overfull lines
\providecommand{\tightlist}{%
\setlength{\itemsep}{0pt}\setlength{\parskip}{0pt}}
\setcounter{secnumdepth}{0}
% Redefines (sub)paragraphs to behave more like sections
\ifx\paragraph\undefined\else
\let\oldparagraph\paragraph
\renewcommand{\paragraph}[1]{\oldparagraph{#1}\mbox{}}
\fi
\ifx\subparagraph\undefined\else
\let\oldsubparagraph\subparagraph
\renewcommand{\subparagraph}[1]{\oldsubparagraph{#1}\mbox{}}
\fi

%%% Use protect on footnotes to avoid problems with footnotes in titles
\let\rmarkdownfootnote\footnote%
\def\footnote{\protect\rmarkdownfootnote}

%%% Change title format to be more compact
\usepackage{titling}

% Create subtitle command for use in maketitle
\newcommand{\subtitle}[1]{
\posttitle{
\begin{center}\large#1\end{center}
}
}

\setlength{\droptitle}{-2em}
\title{}
\pretitle{\vspace{\droptitle}}
\posttitle{}
\author{}
\preauthor{}\postauthor{}
\date{}
\predate{}\postdate{}

\usepackage{setspace}
\usepackage{mathtools}

\begin{document}

\noindent\parbox[t]{0.6\textwidth\large}{\raggedright Hello world \\ Hello world \\ Hello world}\hfill
\parbox[t]{0.3\textwidth\large\onehalfspacing}{\raggedright Hello world \\ Hello world}

\end{document}

• Welcome to TeX.SX! Is there any way you can obtain the complete .tex file that is compiled with TeX? Jun 20 '18 at 19:08
• Thank you! yes, the .tex file is saved in my working directory. I searched for "\let" in the .tex file and found it in these lines:
– fred
Jun 20 '18 at 19:19
– fred
Jun 20 '18 at 19:20
• Does this help somehow?
– fred
Jun 20 '18 at 19:21
• Mhhhh, that doesn't really help me much... On first glance I would have said this is not the problematic code. The \let TeX is complaining about might be hidden somewhere else. Can you try to reproduce the problem in a document that is as small as possible and show us the entire (cut-down) .tex file, please (see also tex.meta.stackexchange.com/q/228/35864) Jun 20 '18 at 19:24

The syntax in your \parboxes is wrong

\noindent\parbox[t]{0.6\textwidth\large}{\raggedright Hello world \\ Hello world \\ Hello world}\hfill
\parbox[t]{0.3\textwidth\large\onehalfspacing}{\raggedright Hello world \\ Hello world}


should probably be

\noindent\parbox[t]{0.6\textwidth}{\large\raggedright Hello world \\ Hello world \\ Hello world}\hfill
\parbox[t]{0.3\textwidth}{\large\onehalfspacing\raggedright Hello world \\ Hello world}


The first mandatory argument of \parbox must be a length and 0.6\textwidth\large is not a length because it contains the font size switch \large. Likewise 0.3\textwidth\large\onehalfspacing is not a length because it contains \large and \onehalfspacing.

Note that the incorrect syntax only causes errors if the calc package is loaded. Otherwise TeX does not complain about the incorrect input and simply executes the command before it typesets the box.