Make sure the nameservers set match those given by Cloudflare for your domain in the Cloudflare dashboard . Check that you have added the correct domain to Cloudflare, it is spelled correctly and that you have selected a plan (Free, Pro or Business). Lastly, you can delete and then add the domain back to Cloudflare to try to fix this issue; this may give you two new nameservers to set at your registrar.