"[Horn Adapter 1/2-in Qty:]"," "
If the first part of the IIF expression returns True, then you want the expression to return a number, but if False, then return an empty string? It could be that you're trying to place string type data where only a number can go.
"[Horn Adapter 1/2-in Qty:]"
I think there should be no quotes around the reference to the field (as there are not in the first part).
Also, you might want to explore your bracketing; thinking that the OR comparison should take place 1st, and as noted, perhaps not to use a string if a number is required for the field.
IIf(([Horn Adapter Make:]="Copper/Plastic" And (([Horn Adapter 1/2-in Qty:]=1 Or [Horn Adapter 1/2-in Qty:]=2))),"[Horn Adapter 1/2-in Qty:]",0)
I would try the number thing first.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.