Error 1061

I can’t join my site I have this error

I’m using https://freepagebuilder.com/

Judging from the error message you got I’d say the domain simply is already in your account.

If 18 hours ago I had an account created in cloudflare but I deleted it at the time, how much would I have to wait?

Can you check whether that domain really does not show up under the account in question any more? Go through the list of domains and double check that.