Hi Alison,
I see you have assigned the number range for 2015 only to NR object 6. Can you please check your document type in ECC and see to which document type in GTS it is mapped to under SPRO Global Trade Services->General Settings-> Assignment of Document type from feeder system.
After identifying the document type, check which NR object is assigned to GTS document type under
SPRO Global Trade Services->General Settings->Define document types.
Regards
Dhilipan