Apex domains cannot have CNAME records.
Question and response
Question
dig use1-ecs.docebomail.com. NS
Answer
use1-ecs.docebomail.com. 5 NS ns-591.awsdns-09.net.
use1-ecs.docebomail.com. 5 NS ns-1469.awsdns-55.org.
use1-ecs.docebomail.com. 5 NS ns-1916.awsdns-47.co.uk.
use1-ecs.docebomail.com. 5 NS ns-409.awsdns-51.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0