Apex domains cannot have CNAME records.
Question and response
Question
dig xgxxrgwpcb.acm-validations.aws. NS
Answer
xgxxrgwpcb.acm-validations.aws. 5 NS ns-1041.awsdns-02.org.
xgxxrgwpcb.acm-validations.aws. 5 NS ns-1665.awsdns-16.co.uk.
xgxxrgwpcb.acm-validations.aws. 5 NS ns-472.awsdns-59.com.
xgxxrgwpcb.acm-validations.aws. 5 NS ns-707.awsdns-24.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0