Subcaption error

Remove the *. begin{subfigure}{0.5textwidth} instead of begin{subfigure}{0.5*textwidth} works perfectly fine.

Placing two subfigures next to each other that both take up half the textwidth will result in an overfull box warning as there will be a small white spce between them. This can be overcome when removing the hfill and when placing a %sign at the end of the first subfigure environment. Alternatively you could also decrease the width of both subfigures.

(Red lines indicate the start of the margins.)

documentclass{article}
usepackage{graphicx}
usepackage{subcaption}
begin{document}
begin{figure}[h]
    begin{subfigure}{0.5textwidth}
            includegraphics[width=textwidth]{example-image-a}
    end{subfigure}
    hfill
    begin{subfigure}{0.5textwidth}
        includegraphics[width=textwidth]{example-image-b}
    end{subfigure}
end{figure}

begin{figure}[h]
    begin{subfigure}{0.5textwidth}
            includegraphics[width=textwidth]{example-image-a}
    end{subfigure}%
%    hfill
    begin{subfigure}{0.5textwidth}
        includegraphics[width=textwidth]{example-image-b}
    end{subfigure}
end{figure}

begin{figure}[h]
    begin{subfigure}{0.45textwidth}
            includegraphics[width=textwidth]{example-image-a}
    end{subfigure}
    hfill
    begin{subfigure}{0.45textwidth}
        includegraphics[width=textwidth]{example-image-b}
    end{subfigure}
end{figure}

end{document}