Apex domains cannot have CNAME records.
Question and response
Question
dig nxzthgqctm.acm-validations.aws. NS
Answer
nxzthgqctm.acm-validations.aws. 5 NS ns-1934.awsdns-49.co.uk.
nxzthgqctm.acm-validations.aws. 5 NS ns-36.awsdns-04.com.
nxzthgqctm.acm-validations.aws. 5 NS ns-702.awsdns-23.net.
nxzthgqctm.acm-validations.aws. 5 NS ns-1241.awsdns-27.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0