A tabularx
needs at least an X
column.
Avoid vertical rules, they add nothing.
Here are four realizations of your table; the first is the wrong one, the next two are extended to the whole text width. The last one is how I'd do it.
\documentclass{article}
\usepackage{tabularx} % needed for the bad example
\usepackage{siunitx,booktabs} % for the better examples
\begin{document}
\begin{table}[!htp]
\centering
\begin{tabularx}{\textwidth}{ c|c|c c|c c }
\hline
\hline
& & \multicolumn{2}{c|}{Dog} & \multicolumn{2}{c}{Bear} \\
\cline{3-6}
& Cat & A & B & A & B \\
\hline
Value 1 & 0.70 & 0.57 & 0.42 & 0.41 & 0.48 \\
Value 2 & 0.10 & 0.051 & 0.07 & 0.03 & 0.022 \\
\hline
\end{tabularx}
\bigskip
\begin{tabular*}{\textwidth}{@{\extracolsep{\fill}\hspace{\tabcolsep}} cccccc }
\hline
\hline
& & \multicolumn{2}{c}{Dog} & \multicolumn{2}{c}{Bear} \\
\cline{3-6}
& Cat & A & B & A & B \\
\hline
Value 1 & 0.70 & 0.57 & 0.42 & 0.41 & 0.48 \\
Value 2 & 0.10 & 0.051 & 0.07 & 0.03 & 0.022 \\
\hline
\end{tabular*}
\bigskip
\begin{tabular*}{\textwidth}{
@{\extracolsep{\fill}\hspace{\tabcolsep}}
l
S[table-format=1.2]
S[table-format=1.3]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.3]
}
\toprule
& & \multicolumn{2}{c}{Dog} & \multicolumn{2}{c}{Bear} \\
\cmidrule(lr){3-4}\cmidrule(lr){5-6}
& {Cat} & {A} & {B} & {A} & {B} \\
\midrule
Value 1 & 0.70 & 0.57 & 0.42 & 0.41 & 0.48 \\
Value 2 & 0.10 & 0.051 & 0.07 & 0.03 & 0.022 \\
\bottomrule
\end{tabular*}
\bigskip
\begin{tabular}{
l
S[table-format=1.2]
S[table-format=1.3]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.3]
}
\toprule
& & \multicolumn{2}{c}{Dog} & \multicolumn{2}{c}{Bear} \\
\cmidrule(lr){3-4}\cmidrule(lr){5-6}
& {Cat} & {A} & {B} & {A} & {B} \\
\midrule
Value 1 & 0.70 & 0.57 & 0.42 & 0.41 & 0.48 \\
Value 2 & 0.10 & 0.051 & 0.07 & 0.03 & 0.022 \\
\bottomrule
\end{tabular}
\end{table}
\end{document}

X
column for atabularx
. However, in your case, atabular
makes more sense ...