Import PO invoice via excel

Hi all, 

I'm processing to import PO invoice into Pending vendor invoice screen. I find out, it could use 03 entity to import PO invoice:

+Vendor invoice header

+Vendor invoice line

+Vendor invoice subline

I added these entity in a single import project and set "HEADERREFERENCE" field is auto generate. However, when I'm process to import, it happen error for Vendor invoice line as:

"Number sequence is not set
Issue exists in auto generation of number sequence
Issue exist in generate staging data
'2' 'Vendor invoice line' record(s) inserted in staging"

Anyone can show me how can I import vendor invoice header and line in a same project. 

Thanks in advance

Chloe