So you will continue to believe that there are people on this forum, I answer you.
I don't know why you are having the problem but, if you did fire and did sink the ship - proceed on patrol.
I never do the training missions myself. I have been playing SH since the first one, so probably don't need it.
I suggest that it is there for you to gain some idea of the game without enemy units trying to kill you. You have been there, done it, no tee shirt. Oh well, move on captain. There's a war on you know?