Hey! I’m commenting just in case others end up here by searching for an answer to the same issue.
like others said, it’s in the interests section of friendly, it has an icon next to it that looks like a checklist with a pen over it. It looks similar to the one on the option to ask about favorite author, but it is a checklist and not what looks like a page of words with a pen over it (the ask about fave author looks very similar).
Anywhoodle, there are things I’ve noticed that seem like they may make a difference in whether the option shows up. At first my sim only attended Afterlife Anonymous meetings in the evening. I was only able to get the option to ask about unfinished business from a single ghost one time at an evening meeting out of 3 total evening meetings my sim attended. The meetings that begin at 10 AM all ended up working perfectly fine.
I also noticed that the ghosts my sim was able to ask had at least a bit of a green bar for relationship, so more than acquaintance, but less than being friends. My sim knew and had a relationship with the ghosts before asking, so I don’t know if that makes a difference overall, or if it’s just a correlation. I know there are interactions that require a specific relationship level, so that’s why I’m thinking that may affect whether or not someone can ask a ghost about unfinished business.
I have a lot of CC added to my game and tried having my sim ask at evening meetings without any CC and that did not make a difference, so I don’t think it’s a CC/mod issue. Once my sim attended morning meetings it seemed to make a difference, along with relationship level potentially having an effect on whether the option is available at all.
Hopefully this is helpful, I haven’t tested it out enough to be sure yet, but it could be worth it to aim to attend morning meetings and have a bit of a good relationship with the ghost you try to ask. I also noticed attendance is better at morning meetings. There were a couple evening meetings my sim attended where no one else showed up at all from 6 pm to 11 pm. My sim was already in Mourningvale before those two meetings began and stayed at the AA location the entire time, so it doesn’t seem to be caused by my sim not waiting long enough for others to show up.