You have an ad saved in your account that was never put through checkout. When you save an ad you will see a message, "Ad Preview. This page shows how your ad will appear in the listings. It is currently saved to your account Checkout", with the Checkout button just below that. You have to go through that checkout process to "buy" the ad, even though it does not cost anything.
If you look at the yellow menu bar at the top of the page you'll see a shopping cart icon with a '1' next to it. That's your ad still in the checkout. Click on it to go there and finish the process.
I will see if we can add something to the message when the ad is saved to make it clearer.