Tides of War

I'm not sure if this is a bug or what. In the third sub training mission i have destroyed the 3 ships, And the merchant ship is save from attack but when i end the mission it say's i have failed the mission what else is there to do. I thought at first the second an third time i looked for a another sub but no such luck. Does anybody have any idea or had this same problem.

Third Sub Mission ..
Which faction ?

It  helps  to  know  !!!