dlookup #name? error (1 Viewer)

I am trying to get the surnames of customers to populate based on the selected product groups, product models and first names. The expression below results in #name? error. What did I do wrong?

Please note that I do not have a primary key in the query. I only have unique records.

Expressions need to be preceded by an equals sign.

However I doubt DLookup would be your solution because it only returns one record. A subform is probably more likely.