When attempting to create a listing, you may run into this error.
It means you've reached the SKU limit for your Subscription Plan. An easy fix for this is to go to your Profile tab and select Subscription Plan from the drop-down.
From here, you can select a higher SKU limit that better suites your business because you've outgrown the old one! A good problem to have.
However, if you are not ready to take the next step, you could delete some of your current listings to make room for the new ones.