Apex domains cannot have CNAME records.
Question and response
Question
dig phamanh.id.vn. NS
Answer
phamanh.id.vn. 5 NS clark.ns.cloudflare.com.
phamanh.id.vn. 5 NS reza.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0