IIF condition Requires three arguments only IIF(Logical condition,then ,else) NOte: IIF formula is not required any END condition where as your IF condition requires the same.
then = (What you want to perform for your logical condition)
Else= Else Condition in case your logical condition is not matching.
so your formula will be
As far as [Category Parameter] existence concern please look into Parameter Section whether that Parameter is exiting there on not if not then recreate the same.