This may be a bit unclear, but you have to complete two missions before you can get the achievment. You will find the first mission by a lake in Saint Denis. The mission is called A Bright Bouncing Boy and is pretty easy. You then have to go to Dover Hill at night. After that mission all you have to do is wait 48 game hours, and then return to the labratory.
Discover the fate of Marko Dragic/Artifical Intelligence achievement (Red Dead Redemption 2)