Custom environment with parameter: You can't use `macro parameter character #' in vertical mode

The definition of

\newenvironment{TextFile}[1]{\ttfamily}{\par}
{
    #1\\[1ex]
    \begin{center}
    \begin{tabular}{|p{0.9\textwidth}|}
    \hline
    \\
}
{ 
    \\ \\
    \hline
    \end{tabular} 
    \end{center}
}

is wrong, at least the parts with { #1\\ }{...}, which are surely meant as the real document body, not something outside after the {\par} statement which is the end code part of the environment.

This way #1 ... etc. is meant as typesetting after the environment definition which must fail unless the definition is wrapped itself in another command or environment.

\documentclass{article}



\newenvironment{TextFile}[1]{\ttfamily%
    #1 \\[1ex]
    \begin{center}
    \begin{tabular}{|p{0.9\textwidth}|}
    \hline
    \\
}{% 
    \\ \\
    \hline
    \end{tabular} 
    \end{center}
    \par
}

\begin{document}

\begin{TextFile}{Foo}
Learning \LaTeXe\ is fun!
\end{TextFile}

\end{document}

Your definition doesn't work because \newenvironment has the following syntax:

\newenvironmnt{<envname>}[<parms>][<opt>]
  {<begin envname>}% \begin{<envname>}
  {<end envname>}% \end{<envname>}

You've used it with no optional arguments (so [<opt>] has been removed) but supplied four mandatory arguments instead of two (the two being {<begin envname>} and {<end envname>}). Any of the arguments specified in [<parms>] are only available within {<begin envname>}, yet you've attempted to use it inside the third mandatory argument, where it doesn't exist.

---

The following approaches ensure that the title and subsequent block stay together and fit within the text block width.

\documentclass{article}

\usepackage{tabularx}

\newenvironment{TextFile}[1]
  {% \begin{TextFile}{#1}
   \par\ttfamily
   \noindent
   % http://tex.stackexchange.com/a/42331/5764
   \tabularx{\linewidth}{|X|}
     \multicolumn{1}{c}{#1} \\[1ex]
     \hline \\
  }{% \end{TextFile}
     \\ \\
     \hline
    \endtabularx
    \par\medskip
  }

\begin{document}

\begin{TextFile}{My Title}
Foo \\
Bar
\end{TextFile}

\end{document}

Another option using listings:

\documentclass{article}

\usepackage{listings}

\lstnewenvironment{TextFile}[1]
  {\lstset{
    basicstyle = \ttfamily,
    frame      = lrtb,
    title      = {\ttfamily #1}}}
  {}

\begin{document}

\begin{TextFile}{My Title}
Foo
Bar
\end{TextFile}

\end{document}