try this
select
Tasks.[Emp_ID],
EMPLOYEE.[Emp_Name],
Department.[Dept_ID],
Department.[Dept_Name],
(Sum(Estimated_Hours)- Sum(Actual_Hours))as Total ,
Status_Id
FROM Tasks
Left Join Employee
ON EMPLOYEE.[Emp_ID] = Tasks.[Emp_ID]
Left JOIN Department
ON EMPLOYEE.Dept_ID = Department.Dept_ID
Where Tasks.Status_Id ='1' Or Tasks.Status_Id ='2'
Group By Employee.[Emp_Name],Tasks.[Emp_ID],
EMPLOYEE.[Emp_Name],
Department.[Dept_ID],
Department.[Dept_Name],
Status_Id
End