I got access to the Shadowlands Beta on Thursday 13th, however I am unable to make posts or reply to posts on the leveling and questing, and the classes and covenants feedback forums.
I have full access to all the other forums.
I have checked that I am in the correct region which is EU. I have spoken to some guildies who got access at the same time, and they are also having the same issue.
Edit: I have also tried logging in and out of the forums, and switched characters.