To find out which packages are really in conflict with each other, let's start by making your code more minimal. Here's a version of your example code in which I removed everything (packages and contents) that are unrelated to the error message you get:
\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{booktabs}
\usepackage{arydshln}
\usepackage{xltabular}
\begin{document}
\begin{xltabular}{\textwidth}{X|X|X}
\toprule
One & abcdef ghjijklmn & 123.456778 \\
\end{xltabular}
\end{document}
If you now change the load order of the first three packages from
\usepackage[table]{xcolor}
\usepackage{booktabs}
\usepackage{arydshln}
to
\usepackage{booktabs}
\usepackage{arydshln}
\usepackage[table]{xcolor}
The above example compiles without any errors. Applying this change to your example code also results in a code that compiles without error messages.
Since you mention "I'm learning how to do nice looking tables": Here's a version of your first table that just makes use of the booktabs package in order to visually separate the individual goals and targets. I also used counters in order to get rid of manual numbering:
\documentclass{article}
\usepackage[margin=1in]{geometry}
\renewcommand{\familydefault}{\sfdefault}
\usepackage{enumitem}
\usepackage{booktabs}
\usepackage{xltabular}
\newcolumntype{L}[1]{>{\hsize=#1\hsize\linewidth=\hsize}>{\raggedright\arraybackslash}X}
\newcounter{goal}
\setcounter{goal}{0}
\newlist{tabenum}{enumerate}{1}
\setlist[tabenum]{label*=\thegoal.\arabic*.,
leftmargin=*,
nosep,
before=\begin{minipage}[t]{\hsize},
after=\end{minipage}}
\newcommand{\mygoalline}[1]{\multicolumn{2}{l}{Goal \refstepcounter{goal}\thegoal\enspace #1}}
\begin{document}
\noindent
\begin{tabularx}{\textwidth}{>{\hangindent=12pt}L{0.35}L{0.65}}
\toprule
%\multicolumn{2}{@{}l}{Goal \refstepcounter{goal}\thegoal\enspace Eradicate Extreme Poverty} \\
\mygoalline{Eradicate Extreme Poverty}\\
\midrule
Target \thegoal.A Halve, between 1990 and 2015, the proportion of the people whose income is less than \$1 a day. &
\begin{tabenum}[series=goal1]
\item Proportion of population below \$1 purchasing power parity (PPP) a day$^a$
\item Poverty Gap ratio [incidence x depth of poverty]
\item Share of the poorest quintile in national consumption
\end{tabenum}
\\
\addlinespace
Target \thegoal.B Achieve full and productive employment and decent work for all, including women and young people &
\begin{tabenum}[resume=goal1]
\item Growth of GDP per person employed
\item Employment to population ratio
\item Proportion of employed people living below \$1 (PP) a day
\item Proportion of own-account and contribution family workers in total employment
\end{tabenum}
\\
\addlinespace
Target \thegoal.C Halve, between 1990 and 2015, the proportion of people who suffer from hunger &
\begin{tabenum}[resume=goal1]
\item Prevalence of underweight children under five years of age
\item Proportion of population below minimum level of dietary energy consumption
\end{tabenum}\\
\midrule
\mygoalline{Achieve universal primary education} \\
\midrule
Target \thegoal.A Ensure that by 2015 children everywhere, boy and girls alike, will be able to complete a full course of primary schooling. &
\begin{tabenum}
\item Net enrollment ratio in primary education
\item Proportion of pupils starting grade 1 who reach last grade of primary education
\item Literacy rate of 15- to 24-year-olds, women and men
\end{tabenum}
\\
\midrule
\mygoalline{Promote gender equality and empower women} \\
\midrule
Target \thegoal.A Eliminate gender disparity in primary and secondary education, preferably by 2005, and in all levels of education no later than 2015 &
\begin{tabenum}
\item Ratios of girls to boys in primary, secondary and tertiary education
\item Share of women in wage employment in the non-agricultural sector.
\end{tabenum}\\
\bottomrule
\end{tabularx}
\end{document}
Best Answer
I found the error. I have added by mistake another
\usepackage{longtable}. I read aboutarydshlnthat it should be added after\usepackage{tabularx} \usepackage{longtable}. I hope it will help someone.