First step would be to realize that there isn't a single forum on this network. Only Q&A sites. There's a big difference, check the about page on any of the sites. (In particular, there are no support forums.)
As for your question, did you try the obvious (i.e. do what the error message tells you)? If yes, you might have a question for Super User. If no, go ahead and try out what the error message tells you first.
If you do post on Super User, please post the complete description of your problem in text, inside your post. Images and external links should only support your question, not be the meat of it. Also make sure you describe what you've tried so far to fix your issue yourself.
You can get an overview of all the sites on the network by visitin the site list. When you find a site that looks like it could be a good fit for your question, read it's "about" page, and its FAQ, to make sure you've got it right.
(And no, the list of related questions in the "Ask a question" page only lists questions from the site you're on. It doesn't list questions from other sites.)