APEX

getting error when running below test GET API through Postman

Rest (GET) API code is:

begin 
 select get_access_token(:ptoken) into v_samp from dual;
 if v_samp = 'Y' then
    ret_msg := GET_STD_FEE_SINGLE_VOUCHER(:p_grn, :p_vouch_no);
    htp.print(ret_msg);
 end if;
exception
   when others then
   ret_msg := dbms_utility.format_error_backtrace||'...'||dbms_utility.format_error_stack;
       if v_samp = 'N' then
          htp.print('Access Denied');
       else
          htp.print(ret_msg);
       end if;
end;

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

function calling in above code (GET API):

create or replace FUNCTION GET_STD_FEE_SINGLE_VOUCHER(p_gr_no IN VARCHAR2, 
                                                     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 
     AND party_user_code = p_gr_no;
 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)
 ) INTO v_master_json
   FROM trans@EDULINKS_253 t,
        parties@EDULINKS_253 p,
        invoice_info@EDULINKS_253 i
  WHERE t.party_user_code = p.party_user_code
    AND t.fee_voch_no = i.fee_voch_no
    AND t.fee_voch_no = p_voch_no;
 SELECT JSON_ARRAYAGG(
   JSON_OBJECT(
     'title' VALUE t.remarks,
     'amount' VALUE t.item_cost
   )
 ) INTO v_detail_json
   FROM trans@EDULINKS_253 t
  WHERE t.fee_voch_no = v_voch_no
    AND t.fee_voch_no = p_voch_no;
  
 -- Construct the final JSON object
 v_final_json := JSON_OBJECT(
   'items' VALUE JSON_OBJECT(
     'master' VALUE v_master_json,
     'detail' VALUE v_detail_json
   )
 );
 RETURN v_final_json;
END "GET_STD_FEE_SINGLE_VOUCHER";
/

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

testing GET API in Postman which returns:

{
    "code": "UserDefinedResourceError",
    "title": "User Defined Resource Error",
    "message": "The request could not be processed for a user defined resource",
    "o:errorCode": "ORDS-25001",
    "cause": "An error occurred when evaluating a SQL statement associated with this resource. SQL Error Code 17166, Error Message: ORA-17166: Cannot perform fetch on a PL/SQL statement: next\r\nhttps://docs.oracle.com/error-help/db/ora-17166/",
    "action": "Verify that the URI and payload are correctly specified for the requested operation. If the issue persists then please contact the author of the resource",
    "type": "tag:oracle.com,2020:error/UserDefinedResourceError",
    "instance": "tag:oracle.com,2020:ecid/af1-aVwMiY5eNcwz7pLH2Q"

}

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

when running directly this GET API code in SQL window, it returns 1 row successfully…

declare
 ret_msg varchar2(3000); 
 test_data CLOB; 
 v_samp  varchar2(1) := 'Y';
 ptoken varchar2(100) := '49d4-a716-4466';
 p_grn varchar2(10) := '51';
 p_vouch_no varchar2(100) := '1499;
begin 
 select get_access_token(ptoken) into v_samp from dual;
 if v_samp = 'Y' then
    ret_msg := GET_STD_FEE_SINGLE_VOUCHER(p_grn, p_vouch_no);
    htp.print(ret_msg);
 end if;
exception
   when others then
   ret_msg := dbms_utility.format_error_backtrace||'...'||dbms_utility.format_error_stack;
       if v_samp = 'N' then
          htp.print('Access Denied');
       else
          htp.print(ret_msg);
       end if;
end;

code directly run as shown but through REST API it is throwing error ??

when changed Source Type to PL/SQL REST Service it is showing only status 200 ok but returns nothing.

please help how to resolve this problem?