Apex domains cannot have CNAME records.
Question and response
Question
dig mta-sts.tech. NS
Answer
mta-sts.tech. 5 NS ns-1703.awsdns-20.co.uk.
mta-sts.tech. 5 NS ns-998.awsdns-60.net.
mta-sts.tech. 5 NS ns-50.awsdns-06.com.
mta-sts.tech. 5 NS ns-1210.awsdns-23.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0