Error in adding custom domain `custom_domain is owned by another account`

I am trying to add custom domain vibhav.festnimbus.com to my netlify site https://vibhav-nimbus.netlify.app but it is showing custom_domain is owned by another account.

The custom domain is successfully configured with Cloudflare as when I visit vibhav.festnimbus.com it shows
vibhav-festnimbus-com-

But I didn’t add this domain to any other site. How can I overcome this issue?

hi there, who manages the DNS for the apex domain festnimbus.com? is it netlify?