I think the reason why you can't post is because you didn't apply as a contributor? I didn't know you wished to post stories or art in this group.
If you wish to post in this group, you need to aply as a contributor. You can tell us more about what you wish to post and how it would fit in this group. If you have examples, you can link them for us. You can read prettymuch all submission rules in this journal:
glimpseoi.deviantart.com/blog/…