Caption for figure into a table - latex

Question

I've been trying all day to put captions to some figures I arranged into a table. First I used table but it was not succesful, and then I found this post https://tex.stackexchange.com/questions/383254/placing-figures-inside-table-with-captions-for-each, where tabularx was used. I got good results with figures into the table, but I failed when I try to add captions using \captionof. Here's my code
\documentclass[aip,amsmath,amssymb,reprint]{revtex4-1}
\usepackage{graphicx}
\usepackage{dcolumn}
\usepackage{bm}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[T1]{fontenc}
\usepackage{mathptmx}
\usepackage{float}
\usepackage{xcolor}
\usepackage{multirow}
\usepackage{float}
\usepackage{tabularx}
\usepackage{capt-of}
\usepackage{tabu}
\begin{document}
\begin{table}
\begin{tabularx}{500pt}{ccc}
\includegraphics[width=0.48\textwidth]{16b_red_cabezas_normalizadas.png} %\captionof{figure}
%{\label{fig:red_all} Functional networks for frequency bands. (a) $\theta$, (b) $\alpha$, (c)
%$\beta$, (d) $\gamma$. Node sizes are proportional to $\langle c_w \rangle$.}
&\hspace{0.5cm}
&\includegraphics[width=0.45\textwidth]{2_biplot_alpha.png} %\caption{\label{fig:biplot} Factorial
%plane for dynamics and structure. Frontal lobe (orange), occipital (green), parietal (cyan), temporal
%(purple).}
\end{tabularx}
\end{table}
\end{document}
This code works perfectly, but when I delete % from the code I get some errors like this:
! Missing \endgroup inserted.<inserted text>\endgroup \end{tabularx}
! Missing \cr inserted.<inserted text>\cr \end{tabularx}
! Missing } inserted.<inserted text>} \end{tabularx}
I don't know what I am doing wrong. Thanks for your help.

Answer 1

The source of your problem is that \captionof needs to make a linebreak after the image and your c type columns don't allow that. You can solve this by using a column type that allows line breaks, e.g. p columns of fixed width or, as you are already using a tabularx, a flexible X column.
Besides this, your table with 500pt is much too wide to fit into onto the paper, let alone into a single column. To get a bit more room, you can use a figure* environment instead that will span over both columns (or table*, but as you are showing figures, this seems not appropriate).
\documentclass[aip,amsmath,amssymb,reprint]{revtex4-1}
\usepackage{graphicx}
\usepackage{dcolumn}
\usepackage{bm}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[T1]{fontenc}
\usepackage{mathptmx}
\usepackage{float}
\usepackage{xcolor}
\usepackage{multirow}
\usepackage{float}
\usepackage{tabularx}
\usepackage{capt-of}
\usepackage{tabu}
\begin{document}
\begin{figure*}
\begin{tabularx}{\textwidth}{XcX}
\includegraphics[width=\linewidth]{example-image-duck} \captionof{figure}{\label{fig:red_all} Functional networks for frequency bands. (a) $\theta$, (b) $\alpha$, (c)
$\beta$, (d) $\gamma$. Node sizes are proportional to $\langle c_w \rangle$.}
&\hspace{0.5cm}
&\includegraphics[width=\linewidth]{example-image-duck} \caption{\label{fig:biplot} Factorial
plane for dynamics and structure. Frontal lobe (orange), occipital (green), parietal (cyan), temporal
(purple).}
\end{tabularx}
\end{figure*}
\end{document}