I can't generate the file due to

Question

I'm trying to generate this file but I'm getting an error that pdflatex-halt-on-error-inter... I added the package \usepackage{graphicx}, but its not working, any idea?

\documentclass[12pt]{article}
\usepackage{amsmath,amsthm, amssymb, latexsym}
\usepackage{tikz}
\makeatletter
\renewcommand*\env@matrix[1][c]{\hskip -\arraycolsep
\let\@ifnextchar\new@ifnextchar
\array{*\c@MaxMatrixCols #1}}
\makeatother 
\usepackage{moodle}
\usetikzlibrary{external} % set this 
\tikzexternalize          % explicitly
\usepackage{graphicx}
\begin{document}
\begin{quiz}{Revisiting Linear Algebra}
\begin{multi}[points=2]{Matrix Form of a Linear System}
Consider the following system
\begin{tikzpicture} % matrix inside of tikzpicture node
\node{
$\begin{matrix}[r]
3.0 x_1    &+2.0 x_2     &+2.0  x_3      & -5.0 x_4 & =8 .0   \
0.6 x_1   &+ 1.5 x_2     &+1.5 x_3       & -5.4 x_4 & =2.7 \

1.2 x_1   & -0.3 x_2      & -0.3 x_3      & 2.4 x_4   &  =2.1 , \

\end{matrix}
$};
\end{tikzpicture}
which may be written as a single vector equation; i.e, $\mathbf{AB}=\mathbf{B}$.\
The $\mathbf{A}$ matrix associated with the system is:
\item*
\begin{tikzpicture}
\node{ $
\begin{bmatrix}[r]
3.0    &+2.0      &+2.0        & -5.0   \
0.6    &+ 1.5      &+1.5        & -5.4   \

1.2    & -0.3      & -0.3      & 2.4 , \

\end{bmatrix}
$};\end{tikzpicture}
\item\begin{tikzpicture}
\node{ $
\begin{bmatrix}[r]
8 .0   \
2.7 \

2.1  \

\end{bmatrix}
$};\end{tikzpicture}

  \item \begin{tikzpicture}
\node{$
\begin{bmatrix}[r]
x_1 &\
x_2& \

x_3 &\

x_4 &\
\end{bmatrix}
$};\end{tikzpicture}

\item

\begin{tikzpicture}
\node{$
\begin{bmatrix}[r]
3.0    &+2.0      &+2.0        & -5.0 &  8.0 \
0.6    &+ 1.5     &+1.5       & -5.4  & 2.7 \

1.2    & -0.3      & -0.3       & 2.4 , & 2.1\

\end{bmatrix}
$};
\end{tikzpicture}

\end{multi}
 \end{quiz}
\end{document}