Minecraft Version

1.20.1

What happened?

I followed the wiki to add just a new slot, and when I add

[[curiosSettings]]
  identifier = "necklace"

for example, The server logs [Thread-2/WARN] [ne.mi.fm.co.ConfigFileTypeHandler/CONFIG]: Configuration file /home/berry/createbb/./world/serverconfig/curios-server.toml is not correct. Correcting all the time.

I just want to add some more slots to fit our cuios stuff and such

Relevant Log Outputs

No response

It seems like you're following the old documentation for the configuration. You'll need to follow the 1.20.1 one located here:
https://docs.illusivesoulworks.com/1.20.x/curios/configuration

The equivalent of what you wrote would be:

slots = ["id=necklace"]

Last question, when I put that on the server, will it also tell the clients about the extra slot?

Yes, it will.