Apex domains cannot have CNAME records.
Question and response
Question
dig sectigodns.com. NS
Answer
sectigodns.com. 5 NS ns-422.awsdns-52.com.
sectigodns.com. 5 NS ns-1836.awsdns-37.co.uk.
sectigodns.com. 5 NS ns-1042.awsdns-02.org.
sectigodns.com. 5 NS ns-925.awsdns-51.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0