I am running into the same issue. I initially tried using "cartIds" as my key. This gave me the same message as the Ravs. However, in another post, for cart based recommendations, I am supposed to use the current item as the key:https://experienceleaguecommunities.adobe.com/t5/adobe-target-questions...

The linked page does not explain why there are 3 values in this metric column. Can you provide more insight?