I followed these 2 steps and it is still not counting that I have completed it.
Step 1. Log completely out of the game for at least 15 minutes. This performs a soft reset and loads your character into a new game instance, refreshing your surroundings, which will allow the missing quest item to appear correctly to your character. If this fails to work, try Step 2.
Step 2. Abandon the quest and re-start it. This will allow the environment to reset and allow the game to respawn the missing object.