I try to run it, I receive ORA-00909: invalid number of arguments. Where did I go wrong?

Display the full names of employees (including title and initial) who manage other employees,
ordered by last name in descending order. Don't forget to include the title, initial, first name, and
last name and look at the data given to determine how to designate those employees who manage
others (i.e., EMP_MGR field is null). [hint: you should have 3 rows of output ordered properly].

 

I am using the following code:

SELECT CONCAT(EMP_TITLE, ' ', IFNULL(EMP_INITIAL, ''), EMP_FNAME, ' ', EMP_LNAME) AS Full_Name
FROM EMPLOYEE
WHERE EMP_NUM IN(SELECT DISTINCT EMP_MGR FROM EMPLOYEE WHERE EMP_MGR IS NOT NULL)
ORDER BY EMP_LNAME DESC;