Hi,
The error suggests the combination of "Vat Bus. Posting Group" of blank and "VAT Prod. Posting Group" of 10% doesn't exist in the VAT Posting Setup page.
In this case I suspect the VAT Business Posting Group on the customer is blank and this has pulled onto the Sales Order.
If you check the option below on the customer and populate it if its blank and then try again:

I hope this helps.
Thanks
Gavin