We’d like to note a difference between the Shadowlands Beta and the live game that you’re going to experience later today.
On your first character to explore the Shadowlands, you will not be able to queue for a Shadowlands dungeon if you haven’t yet restored access to that dungeon’s zone via the main quest campaign for that zone.
For example, while Necrotic Wake will be available the moment you depart Oribos (when you first reach the Shadowlands), you’ll have to complete the Bastion campaign and reopen the way to Maldraxxus before you can queue for the Plaguefall dungeon. You’ll then later unlock Mists of Tirna Scithe and Halls of Atonement as you complete their associated campaigns.
On subsequent characters, all dungeons can be made immediately available to you via the Threads of Fate system.