Here's how the YWP site works: During the official NaNoWriMo season (October and November) we really want people to focus on their official NaNoWriMo projects. That means, you can't create a new personal challenge during those months, just your NaNoWriMo novel.
When the NaNo challenge ends on December 1, then you'll be able to create your own personal challenge from your Dashboard. Your NaNo novel will also be stored on site and you can keep working on it whenever you want.
Desperately want to start a new novel, but it's not December 1 yet? Here's our advice:
1. Raise your word-count goal on your current NaNo novel and focus on writing more of that story! If you've already reached your goal, it probably means your goal was too low, and you need more of a challenge to keep you writing throughout November.
2. If you just have to work on a new story, then for now, just start a new chapter of your NaNo novel and write your new story there. Make it a chapter if you want it to count towards your word-count goal, or make it a note if you don't. Even though NaNoWriMo is technically about writing one novel the whole month, it's okay to be a rebel. One year, I wrote a series of short stories for my NaNo project. I just stored them all in the same doc so their word-count added up together.