Try waiting a few hours, then try again! If you’ve tried to add the website too quickly, then it might be rate-limiting you or something!
Otherwise, the domain maybe suspended for a terms of service (TOS) violation or some other sort of abuse! In this case, you should have gotten an email with the subject “[CLOUDFLARE] [ACTION REQUIRED] Your Cloudflare account has been suspended”! If you see an email with that subject, you should check it out and submit an appeal if you please you are NOT in violation of the terms of service or something that the domain could be flagged for (such as copyright or CSAM)!