Apex domains cannot have CNAME records.
Question and response
Question
dig greencloudvps.com. NS
Answer
greencloudvps.com. 2 NS will.ns.cloudflare.com.
greencloudvps.com. 2 NS arya.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0