Thanks for reaching out and welcome to Netlify’s Support Forums!
I’m showing that the DNS Zone still exist on the account here:
If you have access to that account, you can delete the DNS Zone here. After deleting the DNS Zone you should be able to add the domain to another site.
If you no longer have access to the account, you can verify ownership of the domain by following this Support Guide:
After creating the TXT Record, please let us know and we’ll verify.