LaTeX (Overleaf) says that equation references (eqref) are undefined, but they're apparently ok. What Am I missing?

Question

LaTeX (Overleaf) says that references eq1 and eq:2 are undefined, but they're apparently ok.

What Am I missing here? MWE is provided. Thanks!

\documentclass[twocolumn,book,showpacs,%
  nofootinbib,aps,superscriptaddress,%
  eqsecnum,prd,notitlepage,showkeys,10pt]{revtex4-1}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{dcolumn}
\usepackage{hyperref}
\usepackage{biblatex}
\usepackage[utf8]{inputenc}
\usepackage{dirtytalk}
\usepackage{chngcntr}
\usepackage[bottom]{footmisc} 
\addbibresource{references.bib}
\usepackage{fancyhdr}
\renewcommand\headrulewidth{0pt} 
\pagestyle{fancy}
\fancyhead[RE,RO]{}
\fancyhead[LE,LO]{}
\begin{document}
\title{Test}
\author{Test}
\affiliation{test@test.com}
\begin{abstract}
In this work...
\end{abstract}
\maketitle
\section{Introduction}
As stated in \eqref{eq1}:
\begin{equation}
a+b \label{eq1}
\end{equation}
Equation \eqref{eq:2} states that
\begin{equation}
\label{eq:2}
b + c = x
\end{equation}
\end{document}

Answer 1

Looks like you are throwing too much in ... This reduced code works:

%\documentclass[twocolumn,book,showpacs,   nofootinbib,aps,superscriptaddress, eqsecnum,prd,notitlepage,showkeys,10pt]{revtex4-1}
\documentclass[10pt]{article}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{dcolumn}
\usepackage{hyperref}
\usepackage{biblatex}
\usepackage[utf8]{inputenc}
%\usepackage{dirtytalk}
%\usepackage{chngcntr}
%\usepackage[bottom]{footmisc} 
%\addbibresource{references.bib}
\usepackage{fancyhdr}
\renewcommand\headrulewidth{0pt} 
\pagestyle{fancy}
\fancyhead[RE,RO]{}
\fancyhead[LE,LO]{}
\begin{document}
\title{Test}
\author{Test}
%\affiliation{test@test.com}
\begin{abstract}
In this work...
\end{abstract}
\maketitle
\section{Introduction}
As stated in \eqref{eq1}:
\begin{equation}
a+b \label{eq1}
\end{equation}
Equation \eqref{eq:2} states that
\begin{equation}
\label{eq:2}
b + c = x
\end{equation}
\end{document}

Result:

Looks like there is some unwanted interaction. To locate the problems, I suggest repeating, what I did: strategic "deletion", i.e. commenting-out, of packages.