The government of Catalonia can hold a referendum following the Law. The problem here is that the government of Catalonia is not following even their own regional laws approved by them, not to mention the Spanish Constitution.

The secessionists have even forbidden the opposition to participate in the debates in the Parliament of Catalonia.

Edit to add: it is not the same a country willing to participate in an union and then deciding to leave it, than a movement in a nation (Spain) where some people in there decide they want to transform a part of Spain in an independent nation.

Remember also that Catalonia has never been a nation.