Apex domains cannot have CNAME records.
Question and response
Question
dig cortexi-tm.com. NS
Answer
cortexi-tm.com. 5 NS ns-947.awsdns-54.net.
cortexi-tm.com. 5 NS ns-700.awsdns-23.net.
cortexi-tm.com. 5 NS ns-486.awsdns-60.com.
cortexi-tm.com. 5 NS ns-1504.awsdns-60.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0