ixmu.net is not using Cloudflare, therefore you should not receive that message. Here in the Community, we cannot help with specific account issues. Please open support ticket by going to the support page.
Hi @pengxp1996, the message indicates the domain is already active under a different account. If you’re attempting to do a partial or CNAME setup via a partner, the error indicates you or someone else has already added the domain to Cloudflare via a partner. As @cbrandt mentioned, the community cannot see the details of your account; do you recall adding the domain to Cloudflare via the partner, or perhaps via a different partner or different partner account?