Apex domains cannot have CNAME records.
Question and response
Question
dig ogivalvietnam.com. NS
Answer
ogivalvietnam.com. 5 NS ns-698.awsdns-23.net.
ogivalvietnam.com. 5 NS ns-1668.awsdns-16.co.uk.
ogivalvietnam.com. 5 NS ns-371.awsdns-46.com.
ogivalvietnam.com. 5 NS ns-436.awsdns-54.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0