How to unlock the Knife in the Dark mission?
Solution 1:
In case you have just finished Act I, "you have to recruit Brûz and take over the Nurnen Fortress before you can go elsewhere", according to this post.
Otherwise, because you mention your data installation being stuck, it might be that your game - the Cirith Ungol area, specifically - has not yet been downloaded completely. Give it more time, preferably without playing, and it should continue downloading until it is playable.