Adjustment amount for tax or shipping tax is a negative value. For help fixing this, see http://sellercentral.amazon.co.uk/gp/help/25261
This error occurs when the Tax or ShippingTax value in an adjustments feed (XML) is a negative value. If you uploaded a tab-delimited adjustments file, this error occurs when the item-tax-adj or shipping-tax-adj value is a negative value. These must be a positive value representing the refund amount you want applied to the tax or shipping tax components of an order.
You cannot charge a customer's credit card by supplying a negative value. For more information on charging additional fees to a customer, please see the answer to How can I re-charge a customer's credit card?
To resolve this error resubmit your adjustment data with positive refund values.