aggr() is like an inline table in your expression, with its own expression and dimensions. since you are getting back some kind of column like in a real table, you then are allowed to aggregate over this column. The exact syntax is depending on what you want to achieve and what your expression / dimensions are.
In your above expression, what do you want to achieve with your expression? What is the context of this expression (i.e. in what kind of chart object do you use it, with what dimensions?)?
Maybe you could just put your Due Date > vWorkDate condition just inside your inner sum as additional condition, or maybe you could just remove the outer sum? I don't know, since I don't know your requirement and setting.
A pivot table will use an expression total for your total line. Here you don't get any results, because your if() will not return One single answer two your condition, so the result is undefined (the total line has all due dates linked, so there is no unambiguous answer to the condition). In the straight table, I used the total mode sum of rows to get the correct total, which is not available in a pivot table for any reason.
You could get a sum of rows also in a pivot table, using advanced aggregation (please look into the Help, sum of rows in pivot tables).
But I noticed that you can also include your if() statements into your sums, then the expression total could still be calculated. Please see attached.
There are several things that one could be optimize:
1) Use set expressions instead of the sum(if() ) statements, set expressions should perform much better.
2) Also I think your calculated dimensions could probably be replaced by set analysis in your expression
But first check that current results seems reasonably.
