我想在这个图表的所有箭头上写一些文字。我想写一些希腊字母。但是，当我尝试在花括号中写字时，会出现各种错误。我遗漏了什么？非常感谢！
\begin{figure}
\centering
\begin{tikzpicture}
\node[rectangle, draw=black!60, very thick, minimum size=3mm] (v0) at (-6,0) {Search};
\node[rectangle, draw=black!60, very thick, minimum size=3mm] (v1) at (0,0) {Visite al sito};
\node[rectangle, draw=black!60, very thick, minimum size=3mm] (v2) at (-1.5,1.75) {Video};
\node[rectangle, draw=black!60, very thick, minimum size=3mm] (v3) at (2.7,-1.5) {TV GRPs};
\node[rectangle, draw=black!60, very thick, minimum size=3mm] (v4) at (8,0) {Vendite};
\node[rectangle, draw=black!60, very thick, minimum size=3mm] (v5) at (2.7,1.5) {OOH};
\node[rectangle, draw=black!60, very thick, minimum size=3mm] (v6) at (5,-1.5) {SM Paid};
\node[rectangle, draw=black!60, very thick, minimum size=3mm] (v7) at (-3.5,-1.5) {Display};
\node[rectangle, draw=black!60, very thick, minimum size=3mm] (v8) at (-0.5,-1.8) {Utenti App};
%errori
\node (eps) at(8,1) {$\varepsilon_1$};
\node (delta1) at(-6,1) {$\delta_1$};
\node (delta2) at(0,1) {$\delta_2$};
\node (delta3) at(-1.5,2.75) {$\delta_3$};
\node (delta4) at(2.7,-2.5) {$\delta_4$};
\node (delta5) at(5,-2.5) {$\delta_5$};
\node (delta6) at(2.7,2.5) {$\delta_6$};
\node (delta7) at(-3.5,-2.5) {$\delta_7$};
\node (delta8) at(-0.5,-2.8) {$\delta_8$};
\draw[->] (v0) edge (v1) arc[above] {$a1$};
\draw[->] (v2) edge (v1) {};
\draw[->] (v3) edge (v1) {};
\draw[->] (v5) edge (v4) {};
\draw[->] (v6) edge (v4) {};
\draw[->] (v7) edge (v1) {};
\draw[->] (v7) edge (v0) {};
\draw[->] (v7) edge (v8) {};
\draw[->] (v3) edge (v8) {};
\draw[->] (v8) edge (v1) {};
\draw[->] (v1) edge (v4) {};
\draw[->] (v3) edge (v4) {};
%errori
\draw[->] (eps) edge (v4) ;
\draw[->] (delta1) edge (v0);
\draw[->] (delta2) edge (v1);
\draw[->] (delta3) edge (v2);
\draw[->] (delta4) edge (v3);
\draw[->] (delta5) edge (v6);
\draw[->] (delta6) edge (v5);
\draw[->] (delta7) edge (v7);
\draw[->] (delta8) edge (v8);
\label{fig:example}
\end{tikzpicture}
\caption{Example}
\end{figure}