My site is a whitelabel site, it’s design to use specific Cloudflare nameserver. Now domain was transferred to Cloudflare but I can’t change the DNS nameserver.
Are you trying to change the nameservers of a domain within Cloudflare Registrar? I’m afraid that’s not possible at this time:
6.1 in the Domain Registration Agreement states that you cannot change the nameservers:
6.1 Nameservers . Registrant agrees to use Cloudflare’s nameservers. REGISTRANT ACKNOWLEDGES AND AGREES THAT IT MAY NOT CHANGE THE NAMESERVERS ON THE REGISTRAR SERVICES, AND THAT IT MUST TRANSFER TO A THIRD PARTY REGISTRAR IF IT WISHES TO CHANGE NAMESERVERS.
You’ll either need to use Cloudflare’s DNS and get a list of the required records from your host, or transfer your domain elsewhere to change the nameservers.