A family of Microsoft relational database management systems designed for ease of use.
The expression says:
IF The DueDate is earlier than the current date, then sow Overdue
Else, if the DueDate is equal to or greater than the current date, then show In process.
Else, If the Completed Date isn't Null show Completed
However since the DueDate must be either earlier than the current date or greater/equal it will never get to the third condition.
I would try:
IIF(NOT IsNull(CompleteDate), "Completed", IIF(Duedate<Date(),"Overdue", "InProcess"))
This says if completed date is filled in show Completed,
Else If Duedate earlier than current Date show Overdue
Else show InProcess