Environment undefined ONLY when referencing a preamble.tex

Question

I'm trying to use R Markdown to typeset some notes and I'm having difficulty using the \adjustwidth environment. The issue only arises when I reference some preamble.tex. For instance, if I have a blank preamble.tex, and I run the following, I get an error.

--- 
header-includes: 
   - \usepackage{changepage}
output:   
  pdf_document:
    includes:
      in_header: preamble.tex
---
\begin{adjustwidth}{1cm}{1cm}
Test
\end{adjustwidth}

However, if I don't reference preamble.tex (which is just blank) it works!

--- 
header-includes: 
   - \usepackage{changepage}
output: pdf_document
---
\begin{adjustwidth}{1cm}{1cm}
Test
\end{adjustwidth}

And I don't think it's an issue with preamble.tex because a) it's literally blank b) I can reference it as long as I don't also use \adjustwidth. E.g, the following works.

--- 
header-includes: 
   - \usepackage{changepage}
output:   
  pdf_document:
    includes:
      in_header: preamble.tex
---
Test

Any idea?

Resulting mwe.tex:

\documentclass[]{article} 
\usepackage{lmodern} 
\usepackage{amssymb,amsmath} 
\usepackage{ifxetex,ifluatex} 
\usepackage{fixltx2e} % provides \textsubscript 
\begin{document}
\begin{adjustwidth} 
Test 
\end{adjustwidth} 
\end{document}

Answer 1

I simply do not know r but I can help you with the tex file. Now you can change your r code and compare the result with the mwe I show below.

In your resulting file, let us it name mwe.tex you have to add the call of package changepage to get environment adjustwidth defined. How to do that with r I can not help you, ask for example on page stack overflow and use tag r.

After adding the call to that package environment adjustwidth is defined, but there are two parameters missing for the right and left margin indent.

So you have to change your

\begin{adjustwidth}

to

\begin{adjustwidth}{1cm}{1cm}

So at last you need to get the following compiling code as a result of your r code:

\documentclass[]{article} 
\usepackage{lmodern} 
\usepackage{amssymb,amsmath} 
\usepackage{ifxetex,ifluatex} 
%\usepackage{fixltx2e} % provides \textsubscript 

\usepackage{changepage} % <=============================================
\usepackage{showframe}


\begin{document}
\begin{adjustwidth}{1cm}{1cm} % <=======================================
Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test 
\end{adjustwidth} 
\end{document}

Please see that I commented the call of fixltx2e, because it is not longer needed (added to LaTeX kernel after 2015).

Please see that I only added package showframe to this mwe to visualize the typing area and the margins and the right and lrft indent for adjustwidth.

Then you should get the result:

Answer 2

For anyone interested, I found a way to make it work. For some reason

header_includes:
   - \usepackage{changepage}

wasn't working, but the following did

output:
   - pdf_document: 
      - extra_dependencies: ['changepage']

did!