Do you have a pregnant Sim in your town? In that case the future baby already counts for your Sim limit, so your Sim count is 12/12. If you wait until that baby is born, it will count for the requirements.
It is possible to remove a Sim before completing the Life Dreams and Legacies quest. Tap the Sim and then the red circle next to their portrait. Then you can add a new Sim right away.
Oh, right! I hadn’t thought of that… how silly of me.
Thank you so much for your help! You really saved me — I had no idea what was going on. Now I understand it's because my Sim is pregnant. It all makes sense now!🥰
