Hi Ben,
Thanks for the reply.
I have double checked the documentation and it cleary shows that Sales Credit Note are available for START. I have attached a screenshot showing this.
Perhaps it's meant to say STANDARD instead?
In addition, I have been able to send Credit Notes provided that I use an invoice id as the first error mentioned this. However, online the Credit Note does not seem to be linked to the invoice I specified and cannot be used to a specific invoice either.