PLS-00307 when using json type

The below returns error PLS-00307 on line 9 column 3.

Our DB is 18.11. Can you help?

DECLARE

 jo JSON_OBJECT_T;

BEGIN

 jo := NEW JSON_OBJECT_T;

 jo.put('number',  1);          

  jo.put('string',  'abc');

 jo.put('date',    SYSDATE);

 jo.put('datetime', to_timestamp(SYSDATE));

 jo.put('datetime', systimestamp);

END;