Well, you are missing an $
:) The equation E(P) ...
must be typeset in math mode:
\documentclass{standalone}
\usepackage{amsmath}
\begin{document}
\begin{tabular}{c p{5cm} c}\hline
Attribut & Description & Equation \\ \hline
$E$ & The set of objects that can't be distinguished from each other based on the available attributes & Let $P \subseteq A$ ,
$E(P) = \{\ (x, y) \in U^2 : \forall P, a(x) = a(y) \}$ \\
$ \beta $& Admissible error rate\\
$K$ & Grade level
\end{tabular}
\end{document}
Also, please make sure you include a complete and compilable document with the minimum of packages and definitions necessary to reproduce the problem.