Runaway argument? error with springer template

Question

Why did I get this error, "Runaway argument?"

\RequirePackage{fix-cm}
%
%\documentclass{svjour3}                     % onecolumn (standard format)
%\documentclass[smallcondensed]{svjour3}     % onecolumn (ditto)
\documentclass[smallextended]{svjour3}       % onecolumn (second format)
%\documentclass[twocolumn]{svjour3}          % twocolumn
%
\smartqed  % flush right qed marks, e.g. at end of proof
%
\usepackage{graphicx}
  \begin{document}
    
    \title{Insert your title here%\thanks{Grants or other notes
    %about the article that should go on the front page should be
    %placed here. General acknowledgments should be placed at the end of the article.}
    }
    \subtitle{Do you have a subtitle?\\ If so, write it here}
    
    %\titlerunning{Short form of title}        % if too long for running head
    
    \author{First Author         \and
            Second Author %etc.
    }
    
    \institute{ \at
                   $^{1}$ first address \\
                  
                  %\email{[email protected]}           %  \\
    %             \emph{Present address:} of F. Author  %  if needed
               \and
                \at
                    $^{2}$second address
    }
    
    \date{Received: date / Accepted: date}
    % The correct dates will be entered by the editor
    
    \maketitle
    \begin{abstract}
    \keywords{First keyword \and Second keyword \and More}
    % \PACS{PACS code1 \and PACS code2 \and more}
    % \subclass{MSC code1 \and MSC code2 \and more}
    \end{abstract}
    \end{document}
    % end of file template.tex

Update: How to remove the space between the two lines?

Answer 1

The error is complaining about misplaced \par (blank lines). This runs without error.

\RequirePackage{fix-cm}
%
%\documentclass{svjour3}                     % onecolumn (standard format)
%\documentclass[smallcondensed]{svjour3}     % onecolumn (ditto)
\documentclass[smallextended]{svjour3}       % onecolumn (second format)
%\documentclass[twocolumn]{svjour3}          % twocolumn
%
\smartqed  % flush right qed marks, e.g. at end of proof
%
\usepackage{graphicx}
  \begin{document}
    
    \title{Insert your title here%\thanks{Grants or other notes
    %about the article that should go on the front page should be
    %placed here. General acknowledgments should be placed at the end of the article.}
    }
    \subtitle{Do you have a subtitle?\\ If so, write it here}
    
    %\titlerunning{Short form of title}        % if too long for running head
    
    \author{First Author         \and
            Second Author %etc.
    }
    
    \institute{ \at
                   $^{1}$ first address \\
                  %\email{[email protected]}           %  \\
    %             \emph{Present address:} of F. Author  %  if needed
               \and
                \at
                    $^{2}$second address
    }
    
    \date{Received: date / Accepted: date}
    % The correct dates will be entered by the editor
    
    \maketitle
    \begin{abstract}
    \keywords{First keyword \and Second keyword \and More}
    % \PACS{PACS code1 \and PACS code2 \and more}
    % \subclass{MSC code1 \and MSC code2 \and more}
    \end{abstract}
    \end{document}