Apex domains cannot have CNAME records.
Question and response
Question
dig sinc2030.co.kr. NS
Answer
sinc2030.co.kr. 4 NS cns2.hostcocoa.com.
sinc2030.co.kr. 4 NS cns3.hostcocoa.com.
sinc2030.co.kr. 4 NS cns4.hostcocoa.com.
sinc2030.co.kr. 4 NS cns1.hostcocoa.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0