The specific problem this customer had was that they did not have a <DOCID> tag in the <taSopLineIvcInsert> group tag. This occurs in the SOP Transaction xml file (e.g., SOPTransaction1.xml). An example of the DOCID tag is (not all fields shown):
<taSopLineIvcInsert>
<SOPTYPE>3</SOPTYPE>
<DOCID>CARL02</DOCID>
....
<QUANTITY>10</QUANTITY>
</taSopLineIvcInsert>
On the InvoiceID selected on the Sales Fulfillment Order/Invoice Setup form, the "Allocate by:" selection MUST be set to Document/Batch (not Line Item).
The following website gives additional information:
https://dynamicsgpland.blogspot.com/2014/03/sop-orders-imported-with-econnect-auto.html