Hi, @muumy. I see a support ticket about this on our helpdesk and I’ve already replied there.
For other people wondering about this, in @muumy’s case the custom domain was already added to a different site and team at Netlify. It can only be used once which is why there is an error.
The two solutions for this are to either delete the domain from the other site and team or to use the DNS verification process below:
@muumy, please reply to the support ticket or reply here if you don’t see our reply there.