I would suggest to restrict your query to only find those 2 values.

Select distinct

Case when P.Dept_Value = 'Accounting' Then 'AccountingPerson'

When P.Dept_Value = 'ITServices' Then 'ITPerson'

End Type

From PesonDept P

WHERE P.Dept_Value IN( 'Accounting', 'ITServices')

You're getting nulls because you don't have an ELSE on your CASE.