You can't split a string over multiple lines. A quoted string literal must be all on one line. Try this:
Tbox=annotation('textbox', 'Position', [.13,.39,.17,.085],'String','\alpha \beta \gamma \Gamma \delta \Delta \epsilon \zeta \eta \theta \Theta \iota \kappa \lambda \Lambda \mu \nu \xi \pi \Pi \rho \sigma \Sigma \tau \upsilon \Upsilon \phi \Phi \chi \psi \Psi \omega \Omega',...
'BackgroundColor',[.5,.5,1])
