(Sorry, I replied to this the day after you posted it, but the Reply button didn't work or something. If you didn't get a notification message, sorry! I hope you still have time to write your entry if you want to!)
Yes, it is totally okay to start late. I did. As long as you have around 12,000 words by the time you submit the story, it's fine. And as others have already said, if you end up writing more than 12,000 words, that's great! And you don't have to have each chapter/episode be exactly 400 words:
I hope this helps, and again, I'm so sorry that my previous message wasn't posted as a reply to you!