Apex domains cannot have CNAME records.
Question and response
Question
dig yxmgqtjrrf.acm-validations.aws. NS
Answer
yxmgqtjrrf.acm-validations.aws. 5 NS ns-598.awsdns-10.net.
yxmgqtjrrf.acm-validations.aws. 5 NS ns-1663.awsdns-15.co.uk.
yxmgqtjrrf.acm-validations.aws. 5 NS ns-55.awsdns-06.com.
yxmgqtjrrf.acm-validations.aws. 5 NS ns-1057.awsdns-04.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0