latextools .sublime-project texroot does identify a texroot

Question

I am attempting to build a latex project with included files using LatexTools in sublime text 3. I cannot build a pdf from a subfile. Please tell me how to set up a sublime project to enable pdf building by using ctrl+b while having the editor opened on a child tex file.

Below is a minimal example:

qsar_catalogue.tex:

\documentclass{article}
\title{Exploratory review of QSARs in Toxicology}
\begin{document}
\include{sections/3_methods}
\end{document}

sections/3_methods:

\section{Methods}
test

This stack exchange question mentions that a root tex file can be created by putting the below in your .sublime-project file:

{ "settings": { "TEXroot": "./yourfilename.tex" } }

This should enable tex building from tex files that are included via a \include command. I believe I have done this correctly. If I run check system I get the below key/field:

TeX Root
--------
/home/my_user/path/to/qsar_catalogue.tex

However when I try to build from /home/my_user/path/to/sections/3_methods.tex I get:

entering extended mode
(/home/my_user/path/to/sections/methods.tex
LaTeX2e <2016/02/01>
Babel <3.9q> and hyphenation patterns for 81 language(s) loaded.
! Undefined control sequence.
l.1 \section
            {Methods}
? 

I found more instructions at latextools documentation but after a few hours of changing various settings I had no luck.

Answer 1

r-stein answered this in a comment. I had created my own latex build system and it wasn't functioning correctly. LatexTools build system fixed this problem.

After switching to the correct build system (Latex with the check below) everything worked: