I made a forum on my site. https://ko.libreoffice.org/forum/
But I can't post anything. See this.
And I attempted to make some categories as following this http://userhelp.silverstripe.org/for-website-content-editors/forums/
But I couldn't. see these.
Looking at your tree the forum is marked as yellow (as in staging) perhaps this is one issue. I would also disable 'Show in Categories' if you don't have any categories created.
Do you see (http://www.silverstripe.org/assets/Uploads/silverstripe-forum-error-2.jpg) when you try to add a new category? That looks like you then don't have permission to create new forums. Check your user has edit rights to the page.