This achievement is unlocked upon completing sequence 10, memory 3, The Observatory. Without spoiling anything, this is the memory where you first visit the Observatory.
Unfortunately this is a buggy achievement sometimes does not unlock, and replaying the mission through the progress tracker does not unlock the achievement if you missed it. I recommend following the below strategy to back up your save (edited from a post by Bastian Reader on the forums, thank you for your help
Is there a way to back up saves/save scum on One/Series?)
Bastian Reader said:You can sue a cloud back up save if you’re quick.
1. Play the game normally until just before the mission.
2. Save your game.
3. Now hard close the game. Xbox menu button, quit game. This triggers for your cloud save to be updated.
4. Relaunch the game, load up your save, and trigger the achievement.
If it didn’t unlock, press the Xbox menu button, manage game, click save files, and delete save from Xbox (not cloud). Then relaunch the game and repeat step 4 and hope it unlocks.
As a warning, cloud saves tend to happen every 15 minutes or so. So don’t wait too long to do your steps.
Alternatively, you can do the same steps, but during step 4 you can disconnect your Xbox from your wifi, and play offline, and hope it unlocks when you reconnect online.
If you have 2 xboxs, you can launch the game on one Xbox (best if it’s your home Xbox), sync your save, and then disconnect your wifi. Now you have a save stashed away if there’s an issue. If you need to use it, launch the game on your offline Xbox, then reconnect your wifi.