Apex domains cannot have CNAME records.
Question and response
Question
dig fbnrgqtmtt.acm-validations.aws. NS
Answer
fbnrgqtmtt.acm-validations.aws. 5 NS ns-976.awsdns-58.net.
fbnrgqtmtt.acm-validations.aws. 5 NS ns-416.awsdns-52.com.
fbnrgqtmtt.acm-validations.aws. 5 NS ns-1847.awsdns-38.co.uk.
fbnrgqtmtt.acm-validations.aws. 5 NS ns-1516.awsdns-61.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0