Hi, I am also facing the same problem. I just installed The Sims 4 a couple of days ago, and I am playing on an iMac. I have no mods, and the only expansion I have is the free expansion that provides holiday costumes. Let me describe it in detail:
Early game: In the beginning all the Sims in my household were able to write genre books without a problem. One of my Sims is a freelance writer.
Mid game: This is when the problem starts. Once my freelance writers leveled up his writing skill to level 5 (I think), he got a level 5 gig to write a non-fiction book. I try to select the option Freelance -> Write Book... -> Nonfiction. Nothing happens. No "task icon" is queued up for him to write a book or use a computer. I try to get him to write a genre book from the non-freelance option, but that does not work either. No matter how many times I tell him to, he cannot write any type of genre book. There are other Sims in my household who have the writing skill to write genre books, and the option is visible. Unfortunately, when I click on the option bubble for them to write genre books, just like their freelancing friend, they completely ignore it. No "task icon" for writing or using the computer is queued.
I tried to troubleshoot by moving the Sims' computer, buying a new computer, selling the old computer and then buying a new computer, etc. I also went into the game files and emptied out the cache and cachets file. I copied my save file, deleted "The Sims 4" directory, launched a new game, pasted the save file into the saves directory, and continued my play-through. None of these things fixed the problem,
What I did next: I restarted the game from the beginning and tried again. I attempted 3 separate play-throughs, (with the same Sim household saved in my gallery,) and each time the same thing happened. On one play through, none of my Sims did freelance writing. Instead, I had a Sim just be a normal Writer with fixed work hours. I had him practice writing at home, and eventually he leveled up and tried to write a slightly more advanced book, (may have been nonfiction, I don't remember,) and the same thing happens again. After that point, neither he nor anyone else could write genre books (like before).
On my last play-through, I had my freelancing Sim start writing 4 books of the same name before the bug happened. After the bug happened, he cannot start writing or resume writing. I try to click on "resume writing," and sometimes a "task icon" to write a book is briefly queued, but before the Sim actually writes, the "task icon" disappears, and the Sim still does not write.
My observations: this bug seems to happen after the writer has written a few books. This bug never occurs super early in the game; I have only been able to reproduce it after hours of playing. I think the bug may be triggered by one of the following conditions: after a certain number of books is written, or when a certain genre of book is written, or when a writer reaches a certain skill level. What is notable is that this happened whether or not I had a freelancing Sim.
I hope this is helpful! I would love to see it fixed so that my Sim can finally move on with his writing career!
Thanks,
Wes