The way I understand it, when you send a range from the range tool on the left, this updates the TDC gyro angle based on the AoB and speed settings already in the computer. There is a weird thing where you have to click the send button twice - I saw this in a manual TDC video by Rockin Robbins. I'm not sure why you have to press it twice, or what the first send effect is, but for the computer to update the gyro, you need to press it twice. If you are getting the error message you said, it might be that the computed gyro angle is further than the torpedo allows (it can probably only turn like 90 degrees, or < 180). Hence if the computer is set to a stern shot, and you are trying to shoot at something ahead of you, it might not work. To fix this, switch to a bow tube and reset the gyro by sending the range (twice) again. This will update the computer and you should be able to take your shot.
For example, if I'm being attacked by a DD and I decide to take a 'snap' shot, I'll set the range dial to 400 yards, the speed to zero (and hence AoB doesn't matter) and then I'll just aim ahead of the DD's course (guess where he'll be in like 10 15 seconds), and then send the range twice. This will computer snap shot for an imaginary stopped target at that bearing. This is how you fire at non-moving ships in harbour. As everybody else has said, you don't need to lock a target to fire. The only reason you have to lock is to identify ships, and use the stadimeter tool. In some visibility conditions, you can't lock and hence can't ID and use the stad. But if they're in harbour, you know pretty well how far away they are. You can just dial the range into the TDC dial, and send it when pointing at the stopped ship...this will give you the correct gyro for the shot (but make sure you're on the appropriate torpedo tube - bow/stern).
__________________
|