You can’t post in the root categories, or in #msfs:news-and-announcements, #msfs:official-feedback, #msfs:official-tips-troubleshooting. #su14-beta:beta-news, #gd-commsupport:product-announcements, Bug Reporting Hub > Resolved, Alpha/Beta Archive, Archives.
The root categories are #msfs su14-beta #gd-commsupport or Bug Reporting Hub
In addition, you can also take a look at: Why Can't I Create a Topic?