Spain (executive and judiciary) is choosing to ignore that (unlike Canada, for example), and not letting Catalonia exercise that right in any way. According to polls 70%/80% of Catalans think a referendum about independence should take place. The current Catalan government has the mandate from last elections to find a way for the Catalans to exercise this right even if Spain opposes, given that Spain has also refused all previous proposals to find a solution.
Catalans literally have no other option. They either waive their right to self-determination or they exercise it.
Some relevant sources:
Referendum Law: http://dogc.gencat.cat/ca/pdogc_canals_interns/pdogc_resulta...
The Legitimacy of Catalonia's Exercise of its Right to Decide (a Report by a Commission of International Experts): https://www.unige.ch/gsi/files/9115/0461/7417/EXECUTIVE_SUMM...