Hi Marco,
Avoid using %, assign the result to a symbol and use it.
While[Va < 1.8,
Stato = Inverse[Ma] . ((-Ω^2*Ka) - (Va^2*Fexta));
eigenValues = Eigenvalues[Stato];
a2 = eigenValues[[1]];
b2 = eigenValues[[2]];
Print[a2];
λ[i] = {a2, b2};
i = i + 1;
Va = Va + 0.01;]
