Undefined control sequence begin{document}

documentclass[xcolor=dvipsnames]{beamer}
letTiny=tiny
setbeamercolor{structure}{fg=OliveGreen!50!black}
usetheme{Madrid}

title[Title]{Long Title}
subtitle{Lecture insertlecturenumber : insertlecture}

AtBeginLecture{
setcounter{framenumber}{0}
begin{frame}[plain]
titlepage
end{frame}
}

begin{document}

lecture[LectShort]{LectLong}{1}

begin{frame}
  Text
end{frame}
end{document}

subtitle{Lecture insertlecturenumber : insertlecture}

subtitle{Lecture insertlecturenumber}

documentclass[xcolor=dvipsnames]{beamer}
letTiny=tiny
setbeamercolor{structure}{fg=OliveGreen!50!black}
usetheme{Madrid}

AtBeginLecture{
setcounter{framenumber}{0}
begin{frame}[plain]
titlepage
end{frame}
}

begin{document}
title[Title]{Long Title} %<------ Moved to document body
subtitle{Lecture insertlecturenumber : insertlecture} %<--- Moved to document body

lecture[LectShort]{LectLong}{1}

begin{frame}
  Text
end{frame}
end{document}

documentclass[xcolor=dvipsnames]{beamer}
letTiny=tiny
setbeamercolor{structure}{fg=OliveGreen!50!black}
usetheme{Madrid}

title[Title]{Long Title}

AtBeginLecture{
setcounter{framenumber}{0}
subtitle{Lecture~insertlecturenumber: insertlecture}
begin{frame}[plain]
titlepage
end{frame}
}

begin{document}

lecture[LectShort]{LectLong}{1}

begin{frame}
  Text
end{frame}

begin{frame}
  Text
end{frame}

end{document}