\textit command reports weird error

Question

I have following MWE:

\documentclass[12pt,a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage[color=red,opacity=0.1,contents={}]{background}
\usepackage{hyperref}
\usepackage{indentfirst}
\usepackage{caption}
\usepackage{listings}
\usepackage[listingsutf8]{tcolorbox}
\usepackage{tcolorbox}
\usepackage[slovene]{babel}
\usepackage{titlesec}
\usepackage{float}
\usepackage{pifont}

\setcounter{secnumdepth}{4}

\titleformat{\paragraph}
{\normalfont\normalsize\bfseries}{\theparagraph}{1em}{}
\titlespacing*{\paragraph}{0pt}{3.25ex plus 1ex minus .2ex}{1.5ex plus .2ex}

\author{Marko Frelih\\Firm\\\texttt{email}}
\title{Install Manual}
\date{Ljubljana, Marec 2015}

\begin{document}
\textit{script_turn_hdmi_off.sh}
\end{document}

Now, why I cannot write word script_turn_hdmi_off.sh as italic, since the line with \textit{script_turn_hdmi_off.sh returns error:

! Missing $ inserted.
! Extra }, or forgotten $.
! Missing $ inserted.

What am I missing again?!