They've tried requesting a change and they get told no.
The nation might be defined indivisible but that doesn't stop a referendum taking place, as I posted above. One of the people who wrote the constitution said it wouldn't prevent referendums. Spain won't be torn asunder the moment the result is called.
Surely the best way to do it would be to have a referendum, then decide what to do based on the result. The result based on polling data shows it would be about 41% for independence. The central government can say "okay, we've heard you, let's see what we can do about your devolved powers". Just allowing the same tax-rasing powers that the Basque Country has would probably kill the independence movement overnight.
This cycle of "We're not listening to you till you have a legal referendum - no, we won't allow you to have a legal referendum" is beyond stupid.
Still a lot of open wounds 1939 to 1975. General Franco will be turning in his massive mausoleum.
Spain just swept it under the carpet, completely different from Germany's approach.