# ! Missing number, treated as zero. \let \centering

\section{Azimuthal Angle}
\begin{figure}
\centering
\begin{subfigure}[b][0.3\textwidth]
\centering
\includegraphics[width=\textwidth]{D:/Project/Documentation/Images/muon_phi}
\caption{The azimuthal angle of muons}
\label{muon_phi}
\end{subfigure}
\end{figure}

• Welcome to TeX.SX! How is subfigure defined? Are you using a package like the older subfigure or the newer subfig? Then see my answer. – Heiko Oberdiek Jun 26 '15 at 14:26
• If you're using the subcaption package, please say so. – egreg Jun 26 '15 at 14:34

the subfigure width argument needs curly braces ie

\begin{subfigure}{3.0\textwidth}


however i think that seeing as you have only one image the subfigure environment is somewhat redundant. You can just as easily control position, size, caption numbering etc. using just figure and \includegraphics. (the above solution uses subcaption package)