Apex domains cannot have CNAME records.
Question and response
Question
dig thammyvienxuanhuong.com. NS
Answer
thammyvienxuanhuong.com. 5 NS ns-604.awsdns-11.net.
thammyvienxuanhuong.com. 5 NS ns-1668.awsdns-16.co.uk.
thammyvienxuanhuong.com. 5 NS ns-1494.awsdns-58.org.
thammyvienxuanhuong.com. 5 NS ns-1929.awsdns-49.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0