Can you go into more detail about the triumph? The screenshot is just showing a list but not specifically what the problem is. Edit: if this is referring to the exotic from the legendary triumph, you are missing Commencement on Legendary on your Titan (the character I assume you're doing this on as it has the rest). There is a bug that means completing The Invitation on Legendary also checks off Commencement but that wont count towards the Exotic. Upon completing Commencement, speak to the Altar of Relativity in Caldera and it should award you it.