ORA-01422 error when procedure returns more than one row

The procedure you have created is using a SQL that should only be used if you are expecting to return only a single row. If you are expecting to return more then one row, you should use a cursor. I have added a cursor called c1 (see the second procedure). The second procedure uses a loop to read through the selected records.

CREATE OR REPLACE PROCEDURE testpnet (
   po        OUT   VARCHAR,
   batch     OUT   VARCHAR2,
   voucher   OUT   VARCHAR2,
   invoice   OUT   VARCHAR2
)
AS

BEGIN
   SELECT po_id, grp_ap_id, voucher_id, invoice_id
     INTO po, batch, voucher, invoice
     FROM ps_voucher
    WHERE po_id = '0700185';
   close;
       
   DBMS_OUTPUT.put_line (po);
   DBMS_OUTPUT.put_line (batch);
   DBMS_OUTPUT.put_line (voucher);
   DBMS_OUTPUT.put_line (invoice);
END testpnet;

/************* Cursor **************/

CREATE OR REPLACE PROCEDURE testpnet (
   po        OUT   VARCHAR,
   batch     OUT   VARCHAR2,
   voucher   OUT   VARCHAR2,
   invoice   OUT   VARCHAR2
)
AS

cursor c1 is
   select po_id, grp_ap_id, voucher_id, invoice_id
   from ps_voucher
   where po_id = '0700185'; 

BEGIN

   open c1;
   loop
       fetch c1 into po, batch, voucher, invoice;
       exit when c1%NOTFOUND;
       <do something>
   end loop;
   close;
       
   DBMS_OUTPUT.put_line (po);
   DBMS_OUTPUT.put_line (batch);
   DBMS_OUTPUT.put_line (voucher);
   DBMS_OUTPUT.put_line (invoice);
END testpnet;