I had a similar problem. I waited so long to throw my first party (I was, like, level 46), and when I was finally ready, I couldn't get the game to let me throw the birthday party. It would only say that my sim hadn't picked a trait that week. Don't bother changing traits, that's just a waste of heirloom tickets.
What you'll have to do is either open a new slot and create a new sim, or retire one of your sims and replace them with a child or newly made sim. Then, when they're able to first select a new trait, that's when you can throw the birthday party. I don't know why they have that rule, and why they don't explain this in the tutorial very well, but that's the way it is.
Hope this info helps 😊.