This error is exactly was the error message says it is, transformers can’t loop over list of elements… This occurs when you have a document that can have more than one of the same element (it will show as a document list icon) and you try to map directly from an element within that list to a transformer. This is not possible because the transformers does not know which item in this list you are trying to map from (it can be any list index from [0…n]).
What you can do is not to use a transformer in this case (use the transformer service as a normal service-call instead). If you do it this way I think the mapping from the document to the service call will automatically assume that you are referring to item element . Or you can specify which index you meant by clicking on the mapping-line and setting ‘indices’ properties.
If you are expecting more than one item in this list the obvious option would of course be to use a ‘loop’-step to loop over this list and then use the transformers inside this loop on each element item in the list.