EB Bulk Import Error Message

I am using Document Import Configuration Utility to create an eB document & relate it to a Grouped Virtual Item. The document created OK but not related to the Virtual Item. The following error message "Adding Grouped Virtual Items failed. Not all grouped virtual items are added to the document. Error: Sequence contains no matching element" is logged

Can you tell me what the "Sequence contains no matching element" means?

Steve

I am using eB 15.6.1.115