getting error on creation Oracle JSON Search Index

Question

My Oracle version: 12c 12.1.0.2.0

i want to create a search index for json field. this is the code i run.

CREATE INDEX po_search_idx ON j_purchaseorder (po_document)
  INDEXTYPE IS CTXSYS.CONTEXT
  PARAMETERS ('section group CTXSYS.JSON_SECTION_GROUP SYNC (ON COMMIT)');

result

[Error] Execution (1: 1): ORA-29855: error occurred in the execution of ODCIINDEXCREATE routine
ORA-20000: Oracle Text error:
DRG-12203: section group CTXSYS.JSON_SECTION_GROUP does not exist
ORA-06512: a "CTXSYS.DRUE", line 160
ORA-06512: a "CTXSYS.TEXTINDEXMETHODS", line 366

I think it is necessary to create a section group. how can i do this? I did a search but I don't understand how to create this specific group.