''missing number treated as zero'' error in \maketitle

Question

While writing an article I am getting this error. How do I get rid of it??

The following code reproduces the error message.

\documentclass[aps,pra,showkeys,scriptaddress,
groupedaddress,showpacs,onecolumn,
amsmath,amssymb]{revtex4}
\usepackage[utf8]{inputenc}
\usepackage{braket}
\usepackage{float}
\usepackage{amsmath}
\usepackage{natbib}
\usepackage{tikz}
\usepackage{graphicx}
\usepackage[english]{babel}
\usepackage{dirtytalk}
\usepackage{amsmath} 
\usepackage[utf8]{inputenc}
\usepackage[none]{hyphenat}
\usepackage[colorlinks=true, citecolor=blue]{hyperref}
\usepackage{mathtools}
\usepackage{bm}
\usepackage[a4paper,left=17mm,top=22mm,right=17mm,bottom=15mm]{geometry}
\begin{document}
\sloppy
\allowbreak
\date{\today}
\title{title}

\author{author1 \footnote[\P]{$\P$ These authors have contributed equally in 
this paper}}
\email[]{email}
\affiliation{institute}

\author{author2 \footnote[\P]}
\email[]{email}
\affiliation{institute}

\begin{abstract}
abstract
\end{abstract}
\keywords{Entanglement, Deutsch-Jozsa Algorithm, IBM Quantum Experience}

\maketitle
\onecolumngrid
\section{Introduction}
\end{document}

Answer 1

The following is more of a hacky way to get what you want. Please make sure with the journal editors that this kind of change is allowed.

\documentclass[aps,pra,showkeys,scriptaddress,
groupedaddress,showpacs,onecolumn,
amsmath,amssymb]{revtex4}
\usepackage[utf8]{inputenc}
\usepackage{braket}
\usepackage{float}
\usepackage{amsmath}
\usepackage{natbib}
\usepackage{tikz}
\usepackage{graphicx}
\usepackage[english]{babel}
\usepackage{dirtytalk}
\usepackage{amsmath} 
\usepackage[utf8]{inputenc}
\usepackage[none]{hyphenat}
\usepackage[colorlinks=true, citecolor=blue]{hyperref}
\usepackage{mathtools}
\usepackage{bm}
\usepackage[a4paper,left=17mm,top=22mm,right=17mm,bottom=15mm]{geometry}
\begin{document}
\sloppy
\allowbreak
\date{\today}
\title{Differences between Apples and Oranges}

    \author{Walter Wombat\textsuperscript{\P}\footnotetext{\hspace{-1em}\P These authors have contributed equally in this paper}}
    \email{email of Walter}
    \affiliation{institute of Apples}
    \author{Carl Capybara\textsuperscript{\P}}
    \affiliation{instituteof Oranges}



    \maketitle
    \onecolumngrid
    \section{Introduction}
\end{document}

Answer 2

Please take a look at the differences between the code you posted and the folloging working MWE:

\documentclass[aps,pra,showkeys,scriptaddress,groupedaddress,showpacs,onecolumn,amsmath,amssymb]{revtex4}
\usepackage[utf8]{inputenc}
\usepackage{braket}
\usepackage{float}
\usepackage{amsmath}
\usepackage{natbib}
\usepackage{tikz}
\usepackage{graphicx}
\usepackage[english]{babel}
\usepackage{dirtytalk}
\usepackage{amsmath} 
\usepackage[utf8]{inputenc}
\usepackage[none]{hyphenat}
\usepackage[colorlinks=true, citecolor=blue]{hyperref}
\usepackage{mathtools}
\usepackage{bm}
\usepackage[a4paper,left=17mm,top=22mm,right=17mm,bottom=15mm]{geometry}
\begin{document}
\sloppy
\allowbreak
\date{\today}
\title{title}

\author{author}
\email[]{email}
\affiliation{institute}

\begin{abstract}
%abstract
\end{abstract}
\keywords{Entanglement, Deutsch-Jozsa Algorithm, IBM Quantum Experience}

\maketitle
\onecolumngrid
\section{Introduction} 
\end{document}

The following is the diff of the two MWEs (only different rows are shown):

\title{%title}                            | \title{title}
\author{%author}                          | \author{author}
\email[]{%email}                          | \email[]{email}
\affiliation{%institute}                  | \affiliation{institute}
                                          > \end{document}