When the nameserver option is not available, that usually indicates that another account owns the domain. I can see that the domain is owned by g****v@n**r**s.com on Vercel - if you recognize this email, you may wish to ask them to transfer the domain to your Vercel team: Transferring Domains to Another Team or Project
If you do not recognize this email, please add a TXT record to the apex domain with topic number 4044 as the record value so we can manually verify your domain ownership.