This has recently started happening (possibly due to mod conflicts.) I removed all my CC from the last week and nothing helped. I removed all my CC and I was able to add new sims to households again. The thing is, I have over 1,000 files worth of CC and I really don't feel like removing them and opening the game individually for every file. Does anyone know what kinds of CC could cause this problem? Or maybe if anyone has had this problem, what files you deleted to solve it?
Thanks.

Script mods that need updating would be the first thing I think of. Have you checked SimsVIP.com regarding updated mods? It has been the best so far at listing what is updated and what is not.

If you want, I can take a look at your mods folder - you would need to upload and send a link. I can check it out and run it through Sims 4 Studio (I do a visual check as well as updates). Having the exact mods folder also helps me to see what should and shouldn't be in there.

Nevermind, I found it. It was a super outdated skin overlay that went to the bottom of my CC list because I have my files sorted by date. Thanks for offering the help though

I am going through my daughter's mods folder right now. Watching K-dramas helps as it can be tedious sometimes, but hey, I have no life!! Glad you found it.