The Chantry Plaza is in Celestia, in the District of the Stars. Open you Quest Log file, and look at this quest. In the upper left corner of the quest box, there is a Dialogue Bubble (a question mark). Click on it and you will see what you have already done for the quest, and what you still have to finish.
But it sounds like you need to go back to Celstia, to the District of the Stars, for the Chantry Plaza.