/items = ("stimuli_neutral & stimuli_negative")
/position = (50, 50)
That's simply not valid Inquisit syntax. If you want to refer to an <item> element, you ought to do
/items = nameofitemelement
and you cannot refer to multiple <item> elements as it appears you are trying to do. I recommend you work through the picture IAT tutorial included in the Inquisit documentation for further practical examples.