Listing inside tabularx?

You can use:

\begin{tabularx}{\textwidth}{lX}
 First column &
\begin{lstlisting}^^J
  Sample text^^J
\end{lstlisting} \\
\end{tabularx}

Generally speaking, all verbatim-like commands and environments cannot be directly used in parameters of other commands. In tabularx environments, X columntype works actually as macro arguments(it reads the cell contents into boxes first). listings package partially fixes it, but still has some limits. See "5.1 Listings inside arguments" of manual of listings for more information.

1. tabularx has to read its content before the macros are executed, so it doesn't support verbatim code.

2. Do the listing outside tabularx

   a) Use an external file with \lstinputlisting. This can be done in combination with filecontents to generate this file automatically:

   \documentclass[a4paper,12pt]{scrreprt}
   \usepackage[utf8x]{inputenc}
   
   \usepackage{tabularx}
   \usepackage{listings}
   \usepackage{filecontents}
   
   \newsavebox\mybox
   \begin{document}
   % Writes content to temp file
   \begin{filecontents*}{\jobname.abc}
    Sample text
   \end{filecontents*}
   % Or just add the 'listings' environment here
   % and use `\input` instead.
   
   \begin{tabularx}{\textwidth}{lX}
    First column &
    \lstinputlisting{\jobname.abc} \\
   \end{tabularx}
   
   \end{document} 
   

   b) Store the listing in a box and use the box inside tabularx:

   \documentclass[a4paper,12pt]{scrreprt}
   \usepackage[utf8x]{inputenc}
   
   \usepackage{tabularx}
   \usepackage{listings}
   
   \newsavebox\mybox
   \begin{document}
   \begin{lrbox}{\mybox}
    \begin{lstlisting}
      Sample text
    \end{lstlisting}
   \end{lrbox}
   
   \begin{tabularx}{\textwidth}{lX}
    First column &
     \usebox\mybox \\
   \end{tabularx}
   
   \end{document}