Hi bosum,
the two output options of an expression must be of the same type.
You could try casting the date into a string:
if $currentObject/Plan_StartDate >= $SearchHelper/StartDate
and $currentObject/Planned_EndDate <= $SearchHelper/EndDate
then toString($currentObject/Performance_Plan_ProductionQuantity)
else '0'
Also empty instead of '0' should work then.
Does that help?
Best,
Holger