This is not impossible to organise. Scotland and the UK were in a very similar situation, and determined answers to those issues perfectly well, and held a referendum in which it was closer than expected but Scotland opted to remain part of the UK.

Spain has instead opted to prevent a vote of any kind taking place. Perhaps Spain is different, but I'd imagine that rarely goes well -- it seems to me that the fastest way to get someone to want independence is to tell them they can't have it.

And Spain was a dictatorship until 1978 (still well within living memory). Rushing in to arrest people for printing ballots seems like a way to stir memories of governments Madrid would probably prefer not to be associated with.