Do Mermaids Not Have Traits?!!

Update: I assume it must be a bug, because I was not able to save the household without adding traits, but still had no option to add traits. 🤦 However, I did then have the option to randomize traits, although it still won't show what the randomized traits actually are. I don't want to waste all the work that I put into creating this sim, so my hope is that if I save it with the randomized traits, even though I have no clue what they are, maybe I can go back into CAS later in fulleditmode and change them. Although I hate to do that, because whenever I do, my sims forever continue to get wants and moodlets associated with the original traits. What a pain.

You can only add traits while on the human form, not the supernatural form. So if you were setting her up as a mermaid and picking out her tail, you needed to switch back to the human form before selecting traits.