Apex domains cannot have CNAME records.
Question and response
Question
dig dnsever.co.kr. NS
Answer
dnsever.co.kr. 3 NS ns6.dnsever.co.uk.
dnsever.co.kr. 3 NS ns6.dnsever.net.
dnsever.co.kr. 3 NS ns6.dnsever.com.
dnsever.co.kr. 3 NS ns6.dnsever.co.kr.
dnsever.co.kr. 3 NS ns6.dnsever.org.
Authority
The response has no authority records.
Additional
ns6.dnsever.co.kr. 3 A 15.235.2.100
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0