Hey there, this is happening because the DNS zone you’re trying to use already exists on Netlify under a different account.
If you have some idea of who might own the zone (maybe another one of your accounts, or someone you’ve worked with in the past?), the quickest way to get this resolved is for you to contact that person directly and ask them to remove the zone from their account here: Netlify App
If you’re not sure, please verify domain ownership and let us know when that’s done. If the verification is successful we can delete the DNS zone from the other account so you can use it on yours.