Agent Diagnostics are done at the intro of the game to make you familiar with the controls. Once you reach the end and enter the car, shortly after your achievement will unlock. This won't unlock if you jump into a coop game with a friend at the start of the game. You must begin your own campaign alone to go through the diagnostics. If you miss this, you will need to start a new campaign with a new agent. So if you do, finish everything if you are farther into the game. Then start a new campaign.