You will find your first Scavenger Bot very early in the game, and it is story related, so don't worry, you can't miss it. Now, follow the tutorial to set him on a Scavenge, and when it is ready, an actual on-screen prompt will show up saying "Scavenger Bot waiting at bench". At this point, go to any bench, use it, reclaim your bot, get your materials, and claim your achievement.
You will get your first robot in Chapter 5. You will stumble upon him while climbing down a ladder. This is story related, on every difficulty.