A lawyer says to his client - Unfortunately, that's out of our area of expertise and we cannot further instruct you on/in this situation.
Is one better than the other?
For me, it depends whether you mean they cannot give you instructions on the situation (the legal problem), or because it is outside their area of expertise (=in this situation), they cannot instruct further.
Batman, you have been asked before to give more helpful titles. 'On/in' tells us little. We cannot instruct you on/in ...? would have been a better title.
...and I thought you knew, Batman, that there is no need to write a new post to say Thank you or to quote our replies back to us. Simply click the Thank button on any posts you find helpful. It means that we don't have to open the thread again to read your new post and then find that it doesn't include any new information or an additional question.
It saves everybody's time.