I noticed that the second \addplot does not recognize its xticks, I have tried the solution here but it does not work.

\documentclass{report}
\usepackage{pgfplots}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
width = 0.85*\textwidth,
height = 8cm,
major x tick style = transparent,
ybar=2*\pgflinewidth,
bar width=14pt,
ymajorgrids = true,
ylabel = {Run time speed},
symbolic x coords={Model 3, Model 5, Model 1, Model 4, Model 6, Model 2},
xtick = data,
x tick label style={rotate=90, anchor=east},
bar shift=0pt,
scaled y ticks = false,
enlarge x limits=0.25,
ymin=0,
legend cell align=left,
legend style={
at={(1,1.05)},
anchor=south east,
column sep=1ex
}
]
\addplot[bar shift=0pt]
coordinates {(Model 4, 1.0)
(Model 6, 1.0)
(Model 2, 1.0)};
% This does not recognize x ticks.
\addplot[bar shift=0pt]
coordinates {(Model 3, 1.0)
(Model 5, 1.0)
(Model 1, 1.0)};
\end{axis}
\end{tikzpicture}
\end{document}

symbolic x coords, you can replacextick=databyxtick distance=1,xtickmin={Model 3},xtickmax={Model 2},. – esdd Aug 01 '18 at 14:22