If you create enough sales tax codes to generate VAT declaration boxes based on assosiation of sales tax code only, select in the column Transaction classifier the value Not blank. Alternatively if you use the same sales tax code for different transaction types, select value in the column Transaction classifier. For example, if you use the same sales tax code for sales and purchases, you should have at least four lines for the same sales tax code with Transaction classifier values: Sales, SalesCreditNote, Purchase, PurchaseCreditNote.