There are two instances of "Amdapor": Amdapor Keep and The Lost City of Amdapor. The quest indicates you need Amdapor Keep.
Both of them also have a (Hard) version, which can be unlocked after completing the normal versions.

Of the two dungeons, and their Hard versions, only one instance is not a level 50 dungeon, being The Lost City of Amdapor (Hard). Which isn't relevant, because you need Amdapor Keep.

Without seeing your Duty List, nobody can say for certain. But, still sounds like either you haven't unlocked the correct dungeon, or may need to look closer at your Dungeon list.

To be clear, being a level 50 dungeon, it will be under the "A Realm Reborn" tab of dungeons. You may wish to collapse the "Heavensward" dungeon list to make it easier to see the 15-50 dungeons.