You cannot specify the tablespace in that manner when creating an interMedia index. Instead, you define a preference for storage and then refer to that preference in the 'create index' statement.
begin
ctx_ddl.create_preference('mystorage', 'basic_storage');
ctx_ddl.set_attribute('mystorage', 'i_table_clause', 'tablespace tablespace_name');
ctx_ddl.set_attribute('mystorage', 'k_table_clause', 'tablespace tablespace_name');
ctx_ddl.set_attribute('mystorage', 'r_table_clause', 'tablespace tablespace_name');
ctx_ddl.set_attribute('mystorage', 'n_table_clause', 'tablespace tablespace_name');
ctx_ddl.set_attribute('mystorage', 'i_index_clause', 'tablespace tablespace_name');
end;
create index ... parameters ('storage mystorage');
This process is described very clearly in the interMedia documentation.