it is showing up in singleplayer but not in multiplayer and it is happening to my friend too

It may because you forgot to download the Sangin map that he put at the bottom. I did the same thing at first. It was only in single player and it was messed up, because for whatever reason, it tried to open it in Takistan. When it did that, the FOB's didn't spawn and the vehicles all blew up on entering the game. Double check that you have the correct Sangin map. After i tried it once and it was messed up I went back into Steam and reread all the requirements, and i noticed I was missing the map. After I got the map, I was able to play it in multiplayer, and it worked flawlessly. Let me know if that helps any.