Pretty sure that this would have been your issue, not whether the variables were declared using var or let (which are both acceptable ways to declare your variables).
Sku would have been undefined, therefore iterationData would not exist for that variable hence the error “Cannot read properties of undefined (reading ‘iterationData’)”
I really wish Postman would tell you which line it was erroring on.
It should probably be pm.iterationData.
Are you also aware that you can also retrieve the CSV data by using “data.columnName” instead of pm.iterationData?
let sku = data.Artikelnummer // or
let sku = data["Artikelnummer"]
Using CSV and JSON Data Files in the Postman Collection Runner | Postman Blog