Apex domains cannot have CNAME records.
Question and response
Question
dig zbkrxsrfvj.acm-validations.aws. NS
Answer
zbkrxsrfvj.acm-validations.aws. 5 NS ns-1373.awsdns-43.org.
zbkrxsrfvj.acm-validations.aws. 5 NS ns-1874.awsdns-42.co.uk.
zbkrxsrfvj.acm-validations.aws. 5 NS ns-423.awsdns-52.com.
zbkrxsrfvj.acm-validations.aws. 5 NS ns-631.awsdns-14.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0