Below is the preamble I used. After reading several pages I added abstract but it can't produce abstract my .pdf file.
\immediate\write18{makeindex \jobname.nlo -s nomencl.ist -o \jobname.nls}
\documentclass[paper=letter, fontsize=12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{graphicx}
\usepackage{subfigure}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{fancyhdr}
\usepackage{geometry}
\usepackage{textcomp}
\usepackage{setspace}
\usepackage[font=small]{caption}
\usepackage{float}
\usepackage{lineno}
\usepackage{hyperref}
\usepackage[Glenn]{fncychap}
\setlength{\parindent}{0in}
\onehalfspacing
\usepackage[numbers]{natbib}
\begin{document}
\begin{center}
\textbf{Declaration}\\
\end{center}
%\section*{Abstract}
\begin{Abstract}
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
\end{Abstract}
Now the error message appears is which is very common type:
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
...
l.33 \begin{Abstract}
\end{abstact}and\end{document}at last is missing. To help you show us a complete code resulting in the error you have ... – Mensch May 14 '18 at 12:50bookclass does not provide an environment for the abstract. – campa May 14 '18 at 13:11\addcontentsline{toc}{chapter}{Abstract}. You could also download unnumberedtotoc and use\addchap{Abstract}instead. But if you need only one unnumbered chapter, the above line will be just fine. If the header annoys you, add\markboth{Abstract}{Abstract}as well (both directly after\chapter*{Abst}. – Johannes_B May 14 '18 at 16:56