usepackage{lineno,hyperref, graphicx, caption, color, float, makeidx, amsmath}
 .
 .
 .

 begin{figure}[h]
 centering
 includegraphics[width=linewidth]{fig6.PNG}
 captionsetup{justification=centering}
 caption{...         Afootnotemark \ with dimensions 2^{25} times 2^{25}.}
  label{figure6}
  end{figure}

  footnotetext{url{}}

documentclass[demo]{article}

 usepackage{graphicx, caption, url}

begin{document}
 begin{figure}[h]
 centering
 includegraphics[width=linewidth]{fig6.PNG}
 captionsetup{justification=centering}
 caption{Figure shows comparison of performance for square matrix         Aprotectfootnotemark \ with dimensions $2^{14} times 2^{14}$.}
  label{figure6}
  end{figure}

  footnotetext{url{}}
 end{document}