This achievement is story related. Unless you are subjected to a glitch, you will unlock it upon completing Chapter 10 during your second playthrough. During the Threat Disposal mission, you will have to assist Pascal. After you do so, you will proceed to fight a boss.
Before you head to the boss, go to the nearest access point and make a save--this is insurance in case of a glitch. After the boss battle--it has several phases--you will trigger some cut scenes. When they conclude, you will obtain the second formal ending of the game--Ending B--or not to Be. It will be announced as follows on a blank screen.
NieR: Automata
or not to [B]eImmediately following, the credits roll. As the credits roll, you control 2B in the Bunker with some straight forward play. After the credits, you see a preview. It is about this point that the achievement should unlock. At the end of the Preview, you will be given a chance to save. Say, yes and you will be given a choice of three save slots. If you have not yet gotten this achievement, make sure you do not overwrite the save file you made after assisting Pascal but, instead, use one of the other two slots. After you make your save, you end up at the title screen. Select the save file you just made and you will begin your third playthrough.
If the achievement does not unlock by the end of the Preview, you have probably experienced a glitch. If this happens, clear the cache. To do so, turn off the console by holding the
on the console for at least ten seconds and then unplug the console for at least 20 seconds. Load the save you made after assisting Pascal. Clearing the cache will most likely remedy the problem and you should earn the achievement about the time when the preview starts immediately following the credits.