`\ifx`を含むcontrol sequenceを入れ子にすると、外側の`\ifxxx`と内側の`\else`が結びついてしまうため、エラー

main.tex

% -*- coding: utf-8 -*-
\documentclass[dvipdfmx,uplatex]{jsarticle}
\usepackage[deluxe]{otf}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}

%%%%%

\newcommand{\ifempty}[1]{%
  \edef\tempval{#1}%
  \ifx\tempval\empty%
}

\def\sampleflag{Y}

\newcommand{\samplecommand}{SAMPLE}

\ifempty{\sampleflag}%
  \renewcommand{\samplecommand}[1]{%
    \ifempty{#1}%
      EMPTY
    \else%
      NOT EMPTY   %← ./main.tex:24: LaTeX Error: Missing \begin{document}.
    \fi%
  }%
\fi

%%%%%

\begin{document}
本文\par
\samplecommand
\end{document}