Datediff can cause i can't execute my statement in sql?

I have a query to show some field :

EMPL_KODE | EMPL_NAME | TIME_IN | TIME_OUT | TOTAL_MINUTES | TOTAL_HOURS

====================================================================

 SELECT EMPLOYEE.EMPL_KODE, EMPLOYEE.EMPL_NAME, 
        CAST(ATTANDENCE.DATE_IN + ATTANDENCE.TIME_IN AS TIMESTAMP) AS "TIME_IN", 
        CAST(ATTANDENCE.DATE_IN + ATTANDENCE.TIME_OUT AS TIMESTAMP) AS "TIME_OUT", 
        DATEDIFF(MINUTE,ATTANDENCE.TIME_IN,ATTANDENCE.TIME_OUT) AS "TOTAL_MINUTES",
        DATEDIFF(HOUR, ATTANDENCE.TIME_IN,ATTANDENCE.TIME_OUT) AS "TOTAL_HOURS"
        FROM EMPLOYEE
        JOIN ATTANDENCE
        ON EMPLOYEE.EMPL_KODE=ATTANDENCE.EMPL_KODE
        GROUP BY EMPLOYEE.EMPL_KODE, EMPLOYEE.EMPL_NAME;

It shows error message like this :

Invalid expression in the select list (not contained in either an aggregate function or the GROUP BY clause)

What's wrong with my code ? Can you help me please ?