I am having the same issue. My Sim, Harley, has a lot going for her in terms of traits: Creative, Perfectionist, Creatively Gifted (from childhood), Creative Visionary (from rewards store), and Muser (from aspiration). I will have her read a book in between writing. She is always very inspired when she writes. She has written perhaps ten excellent books, but no bestsellers, which means that she has been stuck at that stage of her aspiration for her entire adult life.
I saw the suggestion to read a book from reference. Unfortunately, the only related trait Harley doesn't have is Bookworm. I could change her traits in the reward store, but it seems odd that this would be a requirement for completing the aspiration. I'm pretty sure it's a bug.
Thanks so much!
PS. I have the Seasons and Get To Work expansion packs. I am on a fairly old Mac with OS 10.11.6
PPS. I have a perhaps related bug which is that if a Sim signs up for a gig and then travels before completing, the completed work is accepted by the client, but the boxes uncheck themselves so the project doesn't count. I wonder if it is related, because Harley kept having this problem as a freelance writer.