@venusdemilosims I remember a long ago sim who wrote a biography for someone without even having to chat with the other sim beforehand. Her relationship with the other sim was very high. My current sim's relationship with the subject of the biography was at 100% before he was offered the opportunity, so I was expecting it to be as easy as that other sim in the past.
Carl's says the skill level for a biography is level 3. It doesn't really make any sense for the game to offer it to a sim when they don't have the skill for it, but his writing skill was at 5 or 6 when I tried to get him started on it. So it shouldn't have been a problem in that respect.
So something seems to just not be working as it should. Maybe it was because I waited so long after he accepted the opportunity before trying to get him started on it. I will wait until he reaches level 10 in the medical career, when hopefully he will be home more. By then his writing skill will be higher too. Then I will try giving him the opportunity with MC, and see if he can manage to do it then.
Thank you, guys for your suggestions.