Multiple Dungeons and Decor Items Unavailable Until Midnight 12.0.1
Originally Posted by Blizzard (Blue Tracker / Official Forums)
Pit of Saron, Seat of the Triumvirate, Skyreach, and Algeth’ar Academy are currently closed on all difficulties in the 12.0.0 pre-expansion update while they undergo changes for Midnight Season 1.

When the 12.0.1 update releases, they’ll reopen on Normal difficulty (except Seat of the Triumvirate, which has no Normal mode), and Heroic and Mythic difficulties will unlock when Midnight Season 1 begins.

Thanks for your patience as we update these dungeons!

Blizzard has confirmed that multiple dungeons will remain inaccessible until the launch of Midnight 12.0.1, which will arrive in the coming weeks. This means that the housing decor items Eye of Acherus from Pit of Saron, Skyreach Circular Table from Skyreach and Valdrakken Bookcase from Algeth'ar Academy cannot be obtained until then. The Argussian Crate decor from Seat of the Triumvirate will remain unavailable until Midnight Season 1 begins on March 17, since the dungeon has no Normal difficulty.