-The CSV Import File should include the column/value for the Item field and mapped to Items : Item (Req) field.
-There is no need to map the Line ID.
-As per the Help Guide Article on Assigning Default Values during Field Mapping:
--Be aware that even if you set a default value for one or more fields in a sublist, no sublist data is imported unless you map at least one field for that sublist.
-See Enhancement # 218052 for other related information.