APEX

POST API Code:
----
declare
 ret_msg varchar2(2000); 
 test_data CLOB; 
 v_samp  varchar2(1) := 'N';
begin 
 select get_access_token(:ptoken) into v_samp from dual;
 if v_samp = 'Y' then
    ret_msg := GET_STD_FEE_SINGLE_VOUCHER(fee_voch_no => :fee_voch_no);
    htp.print(ret_msg);
 end if;
exception
   when others then
       if v_samp = 'N' then
          htp.print('Access Denied');
       else
          htp.print(ret_msg);
       end if;
end

--==========================================================================--

FUNCTION:
---
create or replace FUNCTION GET_STD_FEE_SINGLE_VOUCHER(
   p_voch_no IN VARCHAR2
) RETURN CLOB
IS
 v_master_json      CLOB;
 v_detail_json      CLOB;
 v_final_json       CLOB;
 v_voch_no          VARCHAR2(12);
 v_party_user_code  VARCHAR2(20);
BEGIN
 SELECT DISTINCT fee_voch_no, party_user_code
   INTO v_voch_no, v_party_user_code
   FROM trans@EDULINKS_253
  WHERE fee_voch_no = p_voch_no;
 /* master JSON as CLOB */
 SELECT JSON_OBJECT(
          'fee_voch_no'     VALUE t.fee_voch_no,
          'party_user_code' VALUE t.party_user_code,
          'party_name'      VALUE get_party_name@edulinks_253(t.party_user_code),
          'father_name'     VALUE p.father_name,
          'branch_name'     VALUE get_branch_name@edulinks_253(t.doc_source),
          'class_section'   VALUE get_class_name@edulinks_253(t.class_no)
                             || ' ' || get_section_desc@edulinks_253(t.section_id),
          'session_desc'    VALUE get_session_desc@edulinks_253(t.session_id),
          'fee_month'       VALUE TO_CHAR(t.mon_date,'MON-YYYY'),
          'due_date'        VALUE TO_CHAR(i.due_date,'DD-MON-YYYY'),
          'issue_date'      VALUE TO_CHAR(i.iss_date,'DD-MON-YYYY'),
          'valid_date'      VALUE TO_CHAR(i.valid_till,'DD-MON-YYYY'),
          'class_name'      VALUE get_class_name@edulinks_253(t.class_no),
          'section_desc'    VALUE get_section_desc@edulinks_253(t.section_id)
        RETURNING CLOB
        )
 INTO v_master_json
 FROM trans@EDULINKS_253 t
 JOIN parties@EDULINKS_253 p ON t.party_user_code = p.party_user_code
 JOIN invoice_info@EDULINKS_253 i ON t.fee_voch_no = i.fee_voch_no
 WHERE t.fee_voch_no = p_voch_no;
 /* detail array as CLOB */
SELECT JSON_ARRAYAGG(
          JSON_OBJECT(
            'title'  VALUE t.remarks,
            'amount' VALUE t.item_cost
          )
          RETURNING CLOB
        )
 INTO v_detail_json
 FROM trans@EDULINKS_253 t
WHERE t.fee_voch_no = v_voch_no;
 /* final JSON: embed the already-constructed master/detail JSON using FORMAT JSON */
 SELECT JSON_OBJECT(
          'items' VALUE JSON_OBJECT(
                      'master' VALUE v_master_json FORMAT JSON,
                      'detail' VALUE v_detail_json FORMAT JSON
                    RETURNING CLOB
                    )
          RETURNING CLOB
        )
 INTO v_final_json
 FROM DUAL;
 RETURN v_final_json;
EXCEPTION
 WHEN NO_DATA_FOUND THEN
   RETURN TO_CLOB('{"items":{"master":null,"detail":[]}}');
 WHEN OTHERS THEN
   RAISE;
END GET_STD_FEE_SINGLE_VOUCHER;

--==========================================================================--

when test it using Postman it is showing error

please help, what is the problem and how t can be resolved? how I can RETURN error a meaningful error because it returns RETURN TO_CLOB('{"items":{"master":null,"detail":[]}}'); in case of NO DATA FOUND?
also please guide / ask if anything else require? and if there is better approach achieve this, please help to learn.

note: on compilation of FUNCTION there was no error shown

with best regards