Default value for \addlinespace from booktabs

The \defaultaddspace is a dimension (=length). You have to set it either with: \setlength{\defaultaddspace}{10pt} (LaTeX) or \defaultaddspace=10pt (plainTeX syntax).

The error appears because \defaultaddspace looks for a number (or the optional = + number) but only finds the {.

You would either set it before the table or using \global\defaultaddspace=10pt. It will be grouped inside a table sell and the change is lost when \global is not used.

Note set \defaultaddspace will only be used when \addlinespace is used without the optional argument.

use it this way:

\documentclass{article}
\usepackage{array,booktabs}

\newcommand\myVSpace[1][10pt]{\rule[\normalbaselineskip]{0pt}{#1}}

\begin{document}

\begin{tabular}{|c|p{0.66\textwidth}|}\hline
\myVSpace\textbf{Title} & \textbf{A}\\[\defaultaddspace]
Row & B\\\hline
\myVSpace[20pt]Row & B\\\hline
\end{tabular}

\end{document}

or do not use booktabs and then \\[10pt] instead.

The image shows how the optional argument of \rule works. I used \rule[...]{10pt}{#1} to make the box visible