Well, the error means that with the information provided, qRules was not able to select that field. Looking at your template and your mapping, I can see your "ID" field is inside your repeating group. But in your mapping, you've selected my:myFields as the repeating group. Actually, my:inventory_items is the repeating group.
When you map, then, select my:inventory_items as the repeating group. qRules uses the repeating group XPath + the relative path of "my:ID" to locate the ID node.