How do I post Fanfiction stories(overwrite error)?

You can update existing stories, you just change the word 'properties' to 'content' in the URL. I'm not aware of a way to post new stories. I did hear a rumour that they are removing a tonne of stories because they should be rated "MA" which fanfiction doesn't allow, I don't know if that is true, dear lord I hope not it will take forever, but the rest of fanfic is suffering the same problem. However it is apparently not all categories but I know that Harry Potter, Naruto and Twilight have been mentioned as non-working, I also know from general reading that Twilight and Harry Potter at least do have a hell of a lot of stories that have MA content. HP alone has over 103'000 stories that are rated M.