A Domain Name Server (DNS) for NAT networking could not be determined. Please, check your /etc/resolv.conf for nameserver entries. Either add one manually (man resolv.conf) or ensure that your host is correctly connected to an ISP. If you ignore this warning the guest will not be able to perform nameserver lookups and it will probably observe delays if trying so.
This error occurs when your Virtual Box’s guest try access internet but the main Operating System’s DNS hasn’t been set yet. To fix it, just add your Internet Service Provider (ISP)’s IP to your DNS.
In Ubuntu, the DNS file is /etc/resolv.conf.
Open System > Administration > Network > DNS and add your ISP’s IP to ‘DNS server List Box‘ then refresh the connection.