The Auction House is currently closed to players, with no known ETA for when it will be reenabled.
A short while ago, the Auction House was disabled for players in Dragonflight, prompting an «Internal Auction Error» when accessed as well as a message stating that the Auction House is currently closed. While there is no known current ETA for when the Auction House will be back up, it is possible that Blizzard disabled it in order to address certain items being listed that should not be available to players.
Examples of this are Timerunning Gems as well as The War Within Enchanting materials such as Gleaming Shard.
EDIT: It appears that global outages seem to be occurring for players as well, causing mass disconnects and preventing players from logging in.