Could you please share with us a screenshot of your Mapping tab? Also, I believe the source selection for DAT01 Line Item should be Column header, then in the Line Items tab ensure that the Beginning Inventory is mapped.
Finally, I am curious to see the information in the Details tab of the import result screen. If you scroll all the way to the right, there should be a column with more information about any error or issue encountered.
Einas "Give a Man a Fish, and You Feed Him for a Day. Teach a Man To Fish, and You Feed Him for a Lifetime"