If you have an achievement on your X-Box One that looks like the image shown above, then you’ve been hit by the ‘Done! Unlocking…’ bug. This occurs when you meet the requirements to unlock an achievement but it does not unlock on X-Box Live’s servers. It’s not clear exactly what causes this to happen but there are times when this bug becomes much more widespread and affects many people.
How To Fix It
Over time, every achievement that is in the ‘Done! Unlocking…’ state will unlock. It could take a few hours or it could even take months. I’ve personally had achievements randomly unlock that were stuck in this ‘Done! Unlocking…’ state for over 2 months. When it does unlock, it could be at a completely random time in a completely different game, it just takes time.
If you can’t wait for an achievement to unlock that is stuck, it is sometimes possible to force the achievement to pop by fulfilling the unlock criteria again. This does not always fix the problem though and many achievements will just be stuck this way until they unlock on their own. On average, I’d say about half of the achievements that were stuck in this way for me were able to be unlocked by fulfilling the requirements a second time. It’s probably not worth it to go out of your way to repeat the process if it will take you significant time since it is not guaranteed it will unlock after meeting the requirements for unlocking the achievement again.
In Summary: Your achievement will unlock but it may take some time. It will unlock on its own and does not require you to even launch the game again for it to unlock. It’s an annoying situation to be in but at least it will fix itself over time.