Hey there, this is happening because the custom domain you’re trying to use already exists on Netlify under a different account.
If you have some idea of who might be using the domain (maybe a previous collaborator or an agency 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 domain from their account here:
https://app.netlify.com/sites/SITE-NAME/settings/domain#custom-domains > “Options” dropdown > “Remove domain”
If you’re not sure, please verify domain ownership and let us know by responding here when that’s done. If the verification is successful we can remove the domain from the other account so you can use it on yours.