Hi Will W need a bit more information to POSSIBLY help you. Description of the item you are trying to list and auction duration. Some heavily counterfeited items eBay only allows 5 day or longer listings due to VERO.
More than likely you will have to file a support request through the help page as Auctiva employees have access to the API file and can tell you what is wrong.
There is a possibility someone on here can help if you provide a bit of info though.