I think that this means that you cannot define a default tablespace for an index partition if that partition does not contain subpartitions.
The reason for that would be that the default tablespace for an object is the one to which new partitions or subpartitions would be assigned on creation. If a table or index is not partitioned then of course it could not have a default tablespace for new partitions, and if a table or index partition is not subpartitioned then it cannot have a default tablespace for the addition of new subpartitions.
You probably want to:
alter index
myindex1
modify
default attributes
tablespace myNewIndexTablespace;
... and then rebuild the current index partitions to the new tablespace.
http://docs.oracle.com/cd/E11882_01/server.112/e26088/statements_1010.htm#i2129868