Ideas
luthienrising I just tried what you said to do. I tested it with no mods in the mods folder, deleted the localthumbcache, and the same thing happened. I can't get my sim to turn their imaginary friend into a real sim. The option shows up, I click on it and it looks like was gonna work but the doll disappears a few minutes and comes back. After two or three tries it won't even say the option to turn them into a real sim after a while. I did however yesterday only have the MCCC mod in the mods folder and it did work then. I don't know what could be causing this issue. Someone else I know put it on discord that they are having the same problem as I am with the imaginary friends. So I know I am not the only one.
Rachel31311993 Thanks for doing that testing! I'm moving you to Gameplay Issues for now. Have you tried in a different household or a new save to see if it works in those scenarios?