Simply said, at this point in time you can not sell on eBay. Why depends on your history on eBay... either due to not being a member long enough or for some reason you account has been blocked from allowing you to sell.
some people get a certain amount of FREE listings per month. If thats you, then you maxed-out for this month. Sometimes newer seller have restrictions on selling items in a certain catagory, for some reason. Click below to get eBay's answer
You've either sold too much for this month or earned too much money. Check back often to see when you can next sell :)
You cannot list any more items this month.