Allocate Invoices and credit notes raised 2016 as opening balances for 2017

When I started this in February I wanted to begin from the start of my financial year being 1/1/2017, so I added the sales invoices outstanding and the allocated stage payments received I raised as credit notes, I'm now trying to allocate the credit notes against invoices but cannot, I've tried doing it as you instructed in the link but this does not work, can you help please.

Kind regards Yvonne Sheridan