https://macrumors.zendesk.com/hc/en-us
"I can't post a message in the Forums
In order to post to the Forums, you need to have a registered user account and have activated your account by clicking on the activation link in your email. If you are certain you have done both of these things, there are a few other considerations.
Regular users cannot post new threads in the News and Article Discussion forums:
* MacRumors News Discussion
* MacRumors' Page 2 News Discussion
* MacBytes.com News Discussion
* MacPolls.com Discussion
Only administrators can create new threads in these areas.
The following forums have a minimum post count requirement:
* Marketplace
* Politics, Religion, Social Issues
Only users with a history of 100 posts (user title Regular or higher) can start a thread in the Marketplace forum. Only users with a history of 100 posts can start a thread or post replies in the Politics, Religion, Social Issues forum. "