If you have added as many websites (or any type of ad) that your member level allows for and you delete one or more to make room for other sites, it may say you can't add more websites. If that happens, simply refresh the page to update how many websites you have and to be able to add a new one.

Related articles:

Add a website
Add a banner
Add a text ad
Add a square banner
Add a mini brander
Auto assign
Transfer credits