you cannot do Conditonal Sums at the aggregate level, but rather you have to do it row by row.
you can either use a Running Total do conditionally sum each row or you can create a formula field to assist called 'Null_ID'.
if isnull({Main_Data.ID}) then {Main_Data.Enc Sal Amt}
then Sum the Formula field
RT solution
Name=Null_ID_Sum
field to summarize={Main_Data.ID}
Type=SUM
evaluate=use a formula
isnull({Main_Data.ID})
reset=Never
place in report footer