It looks like the characters that completed Uniting the Isles were created before the WoW license they are currently on, so I’m guessing they were transferred from a different Battle.net account.
My feeling here is that the characters could complete Uniting the Isles because of an account wide achievement, which wouldn’t have transferred with them. Which caused a bit of an issue.
This character is likely the closest to being able to qualify to complete the quest, but looks like you need Nightfallen to friendly still. If you get that, our staff should be able to help.