This error message can appear due to the following common reasons:
1. The domain is not returning a valid SOA.
2. The domain is not returning valid nameservers prior to Cloudflare (you can't just add the Cloudflare nameservers to get around this). The domain must be returning the nameservers active at your registrar **prior to Cloudflare.**
3. Your domain must still be in the process of resolving.
You must contact your registrar for assistance with these items. Cloudflare can not manually add domains in any circumstance.