You have to beat mission 46. I will not talk talk about the mission, but this will explain how to get it (or how I got it) as it's a little confusing.
To unlock this mission you have to do all the main missions up to mission 43. (you do NOT have to do mission 45)
You have to listen to all audio tapes that are yellow (just click on them you don't have to listen to the whole thing.)
You have to complete all yellow side ops.
You do NOT have to complete all side ops (I got it at 47 percent but I bet it could be lower) you do not have to complete any bonus mission (the extremes, total silences or anything else).
You may have to upgrade your mother base a little, but I had one part that was only at level 2, and I still got the mission.
After you finish mission 43, I'd recommend listening to all the audio tapes. Finishing any yellow side ops. Then play side ops, after 3-4 you'll unlock a cutscene back at the base. Play more side ops and you'll get a call unlocking the final mission.
If you're clearing 5 side ops with nothing happening there might be something I missed, but I completed them all (I did not have to do sideop 150 or start the process for mission 45)