Apparently, the achievement has something to do with the B3 Sewer level, which is the very first level in the game. This is the achievement's description:
Complete the quest "A Simple Task" from Technical Officer Ryan.
Problem is: I know where Technical Officer Ryan is, but he doesn't have any quests for me!
I've already completed B3 on every difficulty setting, so it seems that I need to accept this mission before I can complete it.
Is there something I have to get/do/achieve before he'll offer me the Simple Task mission?