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}
letmarvosymLightningLightning
usepackage{wasysym}
letwasysymLightninglightning
usepackage{stmaryrd}
letstmaryrdLightninglightning
usepackage{ulsy}
letulsyLightningblitza
% just for the last two lines:
usepackage{fontspec}

begin{document}
[text{marvosymLightning}text{wasysymLightning}stmaryrdLightningtext{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}

usepackage{ stmaryrd }
lightning
mathmode

usepackage{ wasysym }
lightning
textmode

usepackage{ marvosym }
Lightning
textmode

A better way is to use textup, not text or mbox.

documentclass[11pt,a4paper,oneside]{report} 
usepackage[T1]{fontenc} 
usepackage{fouriernc}
usepackage{mathtools}
usepackage{amssymb,amsthm} 
usepackage[a4paper, hmargin={3.5cm,3cm}, vmargin={2.5cm,2.5cm}]{geometry} %margin
usepackage{ marvosym }

newcommandblitz{textup{Lightning}}

begin{document}
section*{The {Lightning} Symbol in Math Mode is (blitz^{blitz^blitz})}'
begin{equation*}
Lambdagamma blitz_{blitz_blitz}
end{equation*}
end{document}

If you want to use it as an operator, additionally wrap it in mathbin{textup{Lightning}} or mathrel{textup{Lightning}}, etc. If you do not want the symbol to appear bold in bold headers, use textnormal instead of textup. (I don’t believe this does anything for marvosym, but it would for many other symbol fonts.)

If you use text, the symbol will be formatter to match the surrounding text—for example, it will switch to italics if used in an italicized theorem statement. If you use mbox, it will not scale in superscripts or subscripts.

In LuaLaTeX or XeLaTeX, you should use the ☇ (U+260 Lightning) or ⚡ (U+26A1 high voltage) Unicode symbols from a font that contains them, e.g.

documentclass[11pt,a4paper,oneside]{report}
tracinglostchars=2
usepackage{mathtools}
usepackage{amsthm} 
usepackage{unicode-math}

setmainfont{TeX Gyre Schola} % Clone of the New Century Schoolbook font used in fouriernc
setmathfont{TeX Gyre Schola Math}
newfontfamilysymbolfont{DejaVu Sans}[Scale=MatchUppercase]

newcommandtextblitz{{symbolfontsymbol{"26A1}}}
newcommandmathblitz{textup{textblitz}}

usepackage[a4paper, hmargin={3.5cm,3cm}, vmargin={2.5cm,2.5cm}]{geometry} %margin

begin{document}
section*{The {textblitz} Symbol in Math Mode is (mathblitz^{mathblitz^mathblitz}) }
[ Lambdagamma mathblitz_{mathblitz_mathblitz}
]
end{document}