Help me in solving PLSQL41 problem

My issue

i am not able to solve this problem

My code

CREATE OR REPLACE TRIGGER update_department_salary
AFTER INSERT OR UPDATE ON employees
FOR EACH ROW
BEGIN
  -- Calculate the average salary for each department and update department_salary table
  MERGE INTO department_salary ds
  USING (
    SELECT department, AVG(salary) as avg_salary
    FROM employees
    GROUP BY department
  ) e
  ON (ds.department = e.department)
  WHEN MATCHED THEN
    UPDATE SET ds.avg_salary = e.avg_salary
  WHEN NOT MATCHED THEN
    INSERT (ds.department, ds.avg_salary) VALUES (e.department, e.avg_salary);
END;
/

Learning course: Database management systems
Problem Link: Practice - Multiple table udpate in Database management systems