How can I use When-Button-Pressed Trigger PL/SQL to insert and update to the table?

I have created Canvas with fields/text items so that user can fill the request and submit it. While it submit activity, I want the When-Button-Pressed to insert new record and if the record exist then just update record to table. I did created CREATE TRIGGER on that button. When I compiled it and got an error return. Can anyone take a look at it and tell me what did I do wrong? Thank you! Have a great day.  Here's my syntax:

CREATE OR REPLACE TRIGGER Add_RequesOrder
AFTER UPDATE ON tbl_Reqest_Order
REFERENCING NEW AS n
FOR EACH ROW

DECLARE
      
      rowCount NUMBER;

BEGIN            
            SELECT COUNT(*) INTO rowCount FROM tbl_Reqest_Order WHERE Request_ID = :n.Request_ID;

            IF rowCount = 0 THEN                        
                INSERT INTO tbl_Reqest_Order(Request_ID, Submit_By, Request_Date, Site_Location, Req_Priority, Issue_Descript, Resolved_By, Resolved_Descript);
                                    VALUES (:n.Request_ID, :n.Submit_By, :n.Request_Date, :n.Site_Location, :n.Req_Priority, :n.Issue_Descript, :n.Resolved_By, :n.Resolved_Descript);            
            ELSE                  
            UPDATE tbl_Reqest_Order SET tbl_Reqest_Order.Submit_By = :n.Submit_By
                              WHERE tbl_Reqest_Order.Request_ID = :n.Request_ID;            
            END IF;
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