Why Cannot the Protection Mode Be Enabled After a Domain Name Is Connected to WAF?

If a domain name has been added to WAF by others, it cannot be added to WAF again the logged-in user. The domain name ownership is subjected to the user who added it. In this case, add a subdomain name and configure a TXT record for the subdomain name at your DNS provider.

For details, see What Are Impacts If No Subdomain Name and TXT Record Are Configured?