Subfigures with captions don’t stack side by side

Question

I tried to stack two figures side by side, but always come on top of each other with the caption aligned left.

\documentclass[article]{Definitions/mdpi} 

\usepackage{amsmath,amssymb,amsfonts}
\usepackage[utf8]{inputenc}
\usepackage{graphics}
\usepackage[skip=2pt,font=scriptsize]{caption}
\usepackage{caption}
\usepackage{subcaption}
\graphicspath{{Images/}}
\usepackage{array,booktabs,longtable,tabularx,tabulary}
\usepackage{adjustbox}
\usepackage{hhline}

\usepackage{nccmath}
\usepackage{yfonts,color}
\usepackage{ltablex}
\usepackage{amssymb}
\usepackage{wrapfig}
\usepackage{color,soul}
\usepackage{amsfonts}
\usepackage{enumerate}
\usepackage{forest}
\usepackage{tikz-qtree}
\usepackage{tikz}
\usepackage[long]{optidef}
\usepackage{textcomp}
\usepackage{rotating}
\usepackage{tabu}
\usepackage{ltablex}
%\usepackage[export]{adjustbox}
%\usepackage{wrapfig}
%------------------------------

%Table-related commands
%\usepackage{array}
%\usepackage[table]{xcolor}
%\setlength{\arrayrulewidth}{1mm}
%\setlength{\tabcolsep}{18pt}
%\renewcommand{\arraystretch}{1.5}
%\newcolumntype{s}{>{\columncolor[HTML]{AAACED}} p{3cm}}
%-------------------------------------------------------


\title{Positioning tables and figures}
\author{Overleaf}
\date{May 2021}

\graphicspath{{images/}}

\begin{document}

\maketitle

\section{Positioning images}




    \begin{figure}
        %\centering
        \begin{subfigure}{0.49\textwidth}
            \includegraphics[width=0.9\linewidth, height=6cm]{figure1.JPG}

            \caption{}
            \label{fig:uavbuilding}
        \end{subfigure}
        \begin{subfigure}{0.49\textwidth}
            \includegraphics[width=0.9\linewidth, height=6cm]{figure2.jpg} 
            \caption{}
            \label{fig:uavcloseip}
        \end{subfigure}
    \caption{a) System Model to Calculate the $P(LoS)$ Considering the user’s Antenna Height b) A Close-Up of (a) }
    \end{figure}

\end{document}

Answer 1

\documentclass[article]{Definitions/mdpi}

\usepackage{graphicx}
\usepackage[skip=2pt,font=scriptsize]{caption}
\usepackage{subcaption}

\title{Positioning tables and figures}
\author{Overleaf}
\date{May 2021}

\begin{document}
\maketitle
    \begin{figure}[ht]
    \centering
    \begin{subfigure}{0.49\linewidth} % <---- observe "\linewidth>"
        \includegraphics[width=\linewidth, height=6cm]{example-image-duck}
        \caption{}
        \label{fig:uavbuilding}
    \end{subfigure}\hfil%
    \begin{subfigure}{0.49\linewidth}% <---- observe "\linewidth>"
        \includegraphics[width=\linewidth, height=6cm]{example-image-duck}
        \caption{}
        \label{fig:uavcloseip}
    \end{subfigure}
    \caption{a) System Model to Calculate the $P(LoS)$ Considering the user’s Antenna Height b) A Close-Up of (a) }
    \end{figure}
\end{document}

Compiled in Overleaf gives: