The eleventh mission of Metal Gear Solid V does not appear on the list of available missions automatically. Still, it is not too difficult to unlock it. You can try to unlock this one, as soon as there appears mission 82 - Make Contact with Emmerich on the list of available missions. Select this one.
Since side mission 82 takes place in the unexplored part of the map, pick your landing zone to the far West of Serak Power Plant. After you have been deployed, go East. You will be interrupted as soon as you reach Aabe Shifap Ruins marked in the above screenshot. Here, you will be attacked by Quiet and the eleventh main mission will unlock automatically. Your objective here, will be to eliminate the female sniper.