Are you trying to add some sub-domain or domain to your Cloudflare account?
What’s the domain name?
Nevertheless, if you’re the domain owner and have privileges to manage this domain, cross-check with your domain registrar for further troubleshooting and finding a solution.
If you’re an Enterprise plan, wanting to add sub-domain alongside your main (root) domain, then you might have to disable the option which is preventing and protecting you of adding such.