The lightning symbol in an equation environment

Just in order to show some more ways to get a lightning symbol (the answer to your problem has been given in Micos answer and in the referenced questions in my comment):

% arara: lualatex

\documentclass{article}
\usepackage{mathtools}
\usepackage{marvosym}
\let\marvosymLightning\Lightning
\usepackage{wasysym}
\let\wasysymLightning\lightning
\usepackage{stmaryrd}
\let\stmaryrdLightning\lightning
\usepackage{ulsy}
\let\ulsyLightning\blitza
% just for the last two lines:
\usepackage{fontspec}

\begin{document}
\[\text{\marvosymLightning}\text{\wasysymLightning}\stmaryrdLightning\text{\ulsyLightning}\]
% Lua- or XeLaTeX needed for the following example:
Unicode symbol \texttt{U+26A1} "High Voltage" and  \texttt{U+2607} "Lightning"
\setmainfont{DejaVuSans.ttf}
\[\text{\symbol{"26A1}\symbol{"2607}}\]
\end{document}

If you comment out the last package and the last two lines in the document, you may compile with PDFLaTeX as well. I just wanted to show the two unicode symbols I found for lightnings and how you could access them.

Or you use the package fontawesome:

% arara: lualatex

\documentclass{article}
\usepackage{mathtools}
\usepackage{fontawesome}

\begin{document}
\[\text{\faBolt}\]
\end{document}

To instruct LaTeX to treat \Lightning as a text-mode rather than as a math-mode macro, you could type either

\mbox{\Lightning}

or (since you're already loading the amsmath package)

\text{\Lightning}

for all math-mode occurrences of this macro.

---

Addendum: As @egreg has pointed out in a comment, a key issue (bug?) is that the file marvosym.sty provides the instruction

\newcommand{\mvchr}[1]{{\mvs\char#1}}

This macro assumes (implicitly) that the symbols of the marvosym package will only ever be used in text mode. By redefining this macro via

\renewcommand{\mvchr}[1]{\mbox{\mvs\symbol{#1}}}

in the preamble (after loading the marvosym package, of course), the behavior of this macro is modified globally and it's no longer necessary to encase any of the many individual macros of the marvosym package in \mbox wrappers.