Q:

Labelling subfloats in a two-column document

I am writing a two-column document and there are a lot of figures that I need to put in. I had a set of 12 plots and I planned to arrange them in three rows of four.
\begin{figure*}[]
\subfloat[]{ \includegraphics[width = 40mm]{LimitCycle2.jpg}}
\subfloat[]{ \includegraphics[width = 40mm]{s30r19.jpg}}
\subfloat[]{ \includegraphics[width = 40mm]{s30r38.jpg}}
\subfloat[hi \label{fig:hi}]{ \includegraphics[width = 40mm]{s30r41.jpg}}
\caption{This is the second one}
\label{fig:bigsec}
\end{figure*}

There are two more like this. However, I can't seem to reference any of the images. \ref{fig:bigsec} and \ref{fig:hi} both end up compiling to ??.
This has also messed up some of the other figures in my document which were being properly referenced before. All the references to figures on that page have turned into ??.
The figures themselves all display properly, however. 

My use packages section is follows:
\documentclass[reprint, aps, amsmath, amssymb]{revtex4-2}
\usepackage[lofdepth,lotdepth]{subfig}
\usepackage{graphicx}% Include figure files
\usepackage{dcolumn}% Align table columns on decimal point
\usepackage{bm}% bold math
\usepackage[utf8]{inputenc}
\usepackage[table]{xcolor}
\usepackage{physics}
\usepackage{amsmath}
\usepackage{url}
\usepackage[hidelinks]{hyperref}
\hypersetup{breaklinks=true}
\urlstyle{same}
\usepackage[
backend=biber,
sorting=none
]{biblatex}

What am I missing here? I'm getting errors like "Undefined control sequence", "Extra \endgroup", "Extra \endcsname".

A:

Well, with an current MiKTeX 2.9 I can compile the following mwe without errors. And the references are working ... 
The culprit in your given code snippets is that the class loads package natbib and then you can not load/use biblatex! I commented the call of biblatex in the following mwe:
\documentclass[reprint, aps, amsmath, amssymb]{revtex4-2}

\usepackage[lofdepth,lotdepth]{subfig}
\usepackage{graphicx}% Include figure files
\usepackage{dcolumn}% Align table columns on decimal point
\usepackage{bm}% bold math
\usepackage[utf8]{inputenc}
\usepackage[table]{xcolor}
\usepackage{physics}
%\usepackage{amsmath} % <================== not needed, see class option

%\usepackage[% <============ class loads natbib, error with loading biblatex
  %backend=biber,
  %sorting=none
%]{biblatex}

\usepackage{url}
\usepackage[hidelinks]{hyperref}
\hypersetup{breaklinks=true}
\urlstyle{same}

\begin{document}
\begin{figure*}[tb]
\subfloat[]{\includegraphics[width = 40mm]{example-image-a}}
\subfloat[]{\includegraphics[width = 40mm]{example-image-b}}
\subfloat[]{\includegraphics[width = 40mm]{example-image-c}}
\subfloat[hi\label{fig:hi}]{\includegraphics[width = 40mm]{example-image-a}}
\caption{This is the second one}
\label{fig:bigsec}
\end{figure*}

text \ref{fig:bigsec} and \ref{fig:hi} text. % <========================
\end{document}`

and the resulting two pages:

and

