Neither the Spanish government nor the government of the Catalan autonomous community has the legal capacity for a referendum of this type.
The supreme law that governs the Spanish state and to which all subjects and public powers are bound to comply is the Spanish Constitution of 1978.
1.2. La soberanía nacional reside en el pueblo español, del que
emanan los poderes del Estado.
Art.1.2. National sovereignty resides in the Spanish people, from
which emanate the powers of the State.
"Spanish People" has the sovereignty. Not the "government", not "some people", not "an autonomous region Government or Parliament". Any referendum that deals with the integrity of the state has forcibly to be for all Spanish People.
9.1. Los ciudadanos y los poderes públicos están sujetos a la Constitución y al resto del ordenamiento jurídico.
9.1. Citizens and public authorities are subject to the Constitution and the rest of the legal system.
Whatever the process, it must be done according to the constitution and the rest of the laws.
149.1. El Estado tiene competencia exclusiva sobre las siguientes materias:
32.ª Autorización para la convocatoria de consultas populares por vía de referéndum.
149.1. The State has exclusive jurisdiction over the following matters:
32. Authorization for the convening of popular consultations by referendum.
The state (Spanish Government) has the power to authorise a referendum.
So... The Spanish Government is the one to authorise a referendum, but it can't simply because is bound to the Constitution which does not provide for the secession of part of its territory.
- The Constitution is based on the indissoluble unity of the Spanish Nation, common and indivisible homeland of all Spaniards, and
recognizes and guarantees the right to the autonomy of the
nationalities and regions that integrate it and the solidarity between
them.
For such a thing to happen, a Constitution reform should take place according arts. 167 and 168
167
Los proyectos de reforma constitucional deberán ser aprobados por una mayoría de tres quintos de cada una de las Cámaras. Si no hubiera
acuerdo entre ambas, se intentará obtenerlo mediante la creación de
una Comisión de composición paritaria de Diputados y Senadores, que
presentará un texto que será votado por el Congreso y el Senado.
De no lograrse la aprobación mediante el procedimiento del apartado anterior, y siempre que el texto hubiere obtenido el voto favorable de
la mayoría absoluta del Senado, el Congreso, por mayoría de dos
tercios, podrá aprobar la reforma.
Aprobada la reforma por las Cortes Generales, será sometida a referéndum para su ratificación cuando así lo soliciten, dentro de los
quince días siguientes a su aprobación, una décima parte de los
miembros de cualquiera de las Cámaras.
168
Cuando se propusiere la revisión total de la Constitución o una parcial que afecte al Título preliminar, al Capítulo segundo, Sección
primera del Título I, o al Título II, se procederá a la aprobación del
principio por mayoría de dos tercios de cada Cámara, y a la disolución
inmediata de las Cortes.
Las Cámaras elegidas deberán ratificar la decisión y proceder al estudio del nuevo texto constitucional, que deberá ser aprobado por
mayoría de dos tercios de ambas Cámaras.
Aprobada la reforma por las Cortes Generales, será sometida a referéndum para su ratificación.
167
Proposals for constitutional reform must be approved by a majority of three fifths of each of the Houses. If there is no agreement
between the two, an attempt will be made to obtain it through the
creation of a Joint Commission of Deputies and Senators, which will
present a text that will be voted by Congress and the Senate.
If the approval is not obtained by the procedure of the preceding paragraph, and provided that the text has obtained the favorable vote
of the absolute majority of the Senate, the Congress, by a two-thirds
majority, may approve the reform.
Once approved by the Cortes Generales, it will be submitted to a referendum for its ratification when requested, within fifteen days
following its approval, one-tenth of the members of any of the
Chambers.
168
When proposing a total revision of the Constitution or a partial amendment affecting the Preliminary Title, the second Chapter, the
first Section of Title I, or the Title II, the principle shall be
approved by a two-thirds majority of each House , And the immediate
dissolution of the Cortes.
The elected Chambers shall ratify the decision and proceed with the study of the new constitutional text, which shall be approved by a
two-thirds majority of both Houses.
Approved the reform by the Cortes Generales, will be submitted to a referendum for ratification.
TL;DR Why do the Spanish government not organize a national referendum for Catalonia independence?
The Spanish Government can not organize or authorize a referendum that goes against the laws to which it is bound, nor can it renounce a sovereignty that does not belong to it.