A relative had this happen on other sites. In their case it was a number formatting issue. Their fix was to remove any spaces in the postal code field. If your code is like NE4 NE1, try NE4NE1. Also, sometimes these systems are looking for the US five digit post code and reject if more than five numbers. I'm unsure if you can remove the last number(s) or as a workaround enter 00000 or 99999 or even leave blank.