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