Operation $911 is a side operation and you will be able to access it as soon as you have completed Operation "Limp Nudle". When you have completed Operation Limp Nudle, a waypoint will automatically show up on the map, however if you want to carry on with the main story just ignore this waypoint and go onto the next main mission.
If after you've completed the main story mission you want to go back and compete the side mission(s), all you have to do is press the Select button
on your Xbox One controller and use the Right Stick
to navigate over to the app titled "DedSec App". Open that up and use the Right Stick once again to scroll through the different side missions until you find Operation $911. Select that mission to start it, complete all the parts to it (fairly straight forward) and once you've completed everything within the mission, the achievement will unlock.
This is my first guide and I hoped it helped. If you have any problems/issues please reply to this comment/"solution" and I will be happy to help you out.