Please read this topic: Topic limit in Sketchbook Category now enforced
Especially:
If you happen to be one of the users who have more than five topics in the category already, you can ask us moderators to help you merge them, or you have to delete some of your topics and post their content into one of your existing topics.
Your previous postings have exceeded the current maximum number of topics, 5, allowed in the Sketchbook category.