This is a story related achievement. Just continue playing to unlock it.
*** Spoiler - click to reveal ***You'll wake up from your Animus to be greeted by a siren and a lot of people panicking. You'll be told by an Abstergo agent that you need to fix a server core as your session has unleashed a virus into the system. Follow the on screen guides and instructions and you'll get there no problem.
You'll learn how to repair computers while getting this achievement and once you've repaired all twenty you'll unlock this achievement: