Earlier quoted context omitted.
I mean voting for independence, not having an opinion, obviously.
As jayniz says, please find the part of the constitution that outlaws voting for independence. (As opposed to organizing such a vote.)
It's in the article 1.2 of the Spanish Constitution:
> La soberanía nacional reside en el pueblo español, del que emanan los poderes del Estado.
(The national sovereignty resides in the Spanish people, from which emanate the powers of the State.)