The error (Error Code: E1890 Description: The Field DeclaredValue must be between 0.01 and 99999999999.99.) from Royal Mail can be caused when the Price or the Discount for the items on the Order cause the value to become minus. Things to check on the order are:
Does the price of the item(s) match the Order Value?
Is the Discount on the item(s) more than the Price? (Price = 10, Discount = 11)
Is the Discount more than the Order Value? (Order Value = 50, Discount = 60)
Once these changes have been made you will be able to generate the label.