Can you please access the old account, ensure the domain is not only removed from the site, but also from Netlify Domains (via the Domains in the main navigation) then try readding the domain to your new site.
If you no longer have access to the old account, you will need to verify ownership of the domain as outlined in the following support guide:
Once done verifying can you please get back to us if you need still assistance?
welcome to the forums! 