Apex domains cannot have CNAME records.
Question and response
Question
dig xmjnffzjyj.acm-validations.aws. NS
Answer
xmjnffzjyj.acm-validations.aws. 5 NS ns-2001.awsdns-58.co.uk.
xmjnffzjyj.acm-validations.aws. 5 NS ns-1283.awsdns-32.org.
xmjnffzjyj.acm-validations.aws. 5 NS ns-414.awsdns-51.com.
xmjnffzjyj.acm-validations.aws. 5 NS ns-857.awsdns-43.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0