It is as Niek says, being able to post in the Zelda WIP, Other WIP, Completed, and Sponsored board is off limits because at least for Zelda WIP, Other WIP, and Completed when a project is made for that on the site a topic is then automatically created on the forum. I do not understand the juxtaposition of your first and second posts. The first makes mention of how you cannot create a new topic, where as the second one calls into question the idea of wanting to create a new topic on the basis that the new topic is just an update to an old project.
Edit: Never mind, I saw you figured that out. It would probably be for the best to sometime include mention of how stuff isn't directly able to be posted the boards; like in the rules, notification topic, or board description.